The layout is already shaping up very well Chaz- excited to see what comes next.

Always love reading eloquently written, well-thought out posts such as yours, Terence, and I really hope someone influential at Bachmann is taking a look at this forum every once in a while!

Now, for your questions:
1. I can say without a shadow of a doubt that I would gleefully purchase a Bachmann Stepney. I have never been as eager to (presumably) purchase an engine as I would be for Stepney with the one exception of Oliver, who I preordered nearly two years before his release. I would even go so far as to say that if my budget allowed, I'd be willing to purchase TWO models of Stepney, one to run on the circle of track I currently have and one to remain unopened as a shelf piece. That is how committed I am to the production of a Bachmann Stepney...
2. I've previously spoken on the probaility of a Stepney model being popular amongst not only fans but also serious collectors, rail enthusiasts, and even casual collectors. For all the reasons you outlined I cannot see a market where Stepney wouldn't be a hot commodity and I truly think Bachmann is shooting themselves in the foot by not producing Stepney and tapping into a highly profitable revenue stream.
3. I think unless the hand of Mattel says otherwise, Bachmann has no other viable choice but to produce Stepney as the next SG engine. The only possible contender who might precede him would be Boco, but due to Daisy's announcement (long green diesel) and the limited support I've seen for him, I highly doubt they would make that decision.

As usual Chaz has articulated a popular fan opinion in both a detailed and well-researched manner. I tried getting into the narrow gauge line years ago when they first announced Skarloey but the price barrier ended up prohibited me from being able to continue collecting the range, but given what I saw with Skarloey I feel comfortable to give my opinion on the argument in favor of Sir Handel's introduction into the range next. The points made by Chaz are pretty coherent, so I'd like to address his questions instead. Personally, I don't know if I would purchase a Sir Handel model, but again I don't collect the range so my personal opinion here should be taken with a grain of salt. The only NG engine I would ever consider/am considering purchasing is Peter Sam due to a personal and intimate connection that I have had with him since I was young. I think that making any other NG engine next instead of Sir Handel would be a huge misstep for Bachmann. I think even a choice like Duncan would be a mistake given the egregious errors in his CG render. A CG engine would be even worse imo as I think Bachmann should at least finish the original fleet of engines before even considering someone like Luke or Victor. I think that Sir Handel would be incredibly popular for the reasons stated by Chaz but particularly because of his color scheme, which may be a major selling point for more casual consumers looking into the range. Blue is the color of Thomas, after all!
Anyway, I think Sir Handel is a no-brainer for the next engine to be produced for the range, but before we think about him, I wanna see Peter Sam!

Here's the face I'd personally choose:

https://vignette.wikia.nocookie.net/ttte/images/f/f1/RustytotheRescue62.png/revision/latest?cb=20180831173112

I'm honestly appalled, but Bachmann just confirmed on Twitter that they discontinued both Knapford Station and Tidmouth Sheds. I really hope that they have plans to replace them or something, because honestly with the discontinued resin line and now this, the availability of detailed Thomas buildings is dwindling quickly.

That's fair, although Mattel isn't shy with dumb business decisions as we've seen already...

As much as I hate to say it, I can almost guarantee we will see a Rebecca model before we see Hiro. As little sense as that makes from a consumer standpoint, I'm sure Mattel will force Bachmann's hand eventually. And of course with that comes the fact that if Bachmann is made to expend their resources and time on two completely new models, one of which having an insanely complicated valve gear and odd shape overall and the other being a large tender engine, other announcements will be scarce for at least a year, maybe two. I mean, it took two years before we got any really good, new announcements after Bachmann announced Oliver and Skarloey. Wasn't it that one year we got the red coaches and the other Arry and Bert? If it were any other brand (looking at Lionel as an example) I wouldn't be as worried. But given the breadth and variety of the Bachmann range it is honestly only a matter of time before the two key missing steam team members are brought to the range. The fact that Red Rosie was such a priority also stands as proof of this.

Here's my ideal list for what I'd like to see from Bachmann in the future in all of their various lines. I'm guessing anything they have planned for July's NMRA has been green-lit already or is at least in the later stages of development/planning so I'm not sure how much our input matters for those particular announcements but clearly our feedback has an impact on their choices in the long-term so I want to make it super clear what I'd like to see personally in the future, even if we don't see that feedback considered until next year or beyond. These choices are fairly similar to others who have already shared their thoughts, but I figured I'd post them regardless.

HO
Stepney- An obvious choice. There's literally no reason for Bachmann NOT to make him at this point. He's a simple, small, 0-6-0 engine that would not require a complicated tooling or chassis and he is clearly the most highly-desired character at this point. The exorbitantly high prices of Hornby Stepney on eBay are a clear indicator of the demand for a HO scale rendition of this timeless character. Please, Bachmann, please! You answered our prayers for Oliver and the NG line, surely you can consider Stepney as well! The consumer is always right!!!

Ryan- Another no-brainer and a choice that I'm sure Mattel would support. Ryan's design is similar to Stepney in that he is a tank engine with a fairly simple mold design and wheel arrangement (I could easily see them re-using Duck or Rosie's wheels with new side rods and Gordon's tailing bogey for the chassis), plus his color is extremely unique, and given what we've seen with Busy Bee James, clearly the color of the models holds weight in Bachmann/Mattel's eyes. A purple engine would stand out tremendously well in the current line and I am sure we will see Ryan included into the range eventually.

Black James- I am incredibly disappointed in whichever Mattel executive presumably nixed Black James in favor for Busy Bee James (confession: I am still considering purchasing BBJ, because there is something oddly charming and alluring about the pure novelty of it ), because as I've said previously, had they released the LBSC Thomas AND Black James at the same time, Bachmann would have almost certainly doubled their profits as I cannot imagine there would have been many fans who would've bought one without the other. Hopefully Bachmann can address this lapse in judgement and push through what I am assuming was pushback from Mattel regarding the release of this rendition of James. However, I do have to concede that this option would be fairly odd to announce after the 75th anniversary given its commemorative nature, so here's hoping they see our pleas and green-light this simple recolor for the NMRA announcements.

Boco- We have seen that Bachmann is more than willing to consider long, double-bogied diesels for the Thomas range (see: Daisy), so I don't think that Boco is a completely unrealistic expectation. However, I have not seen nearly the same amount of fan support for him, and I think that his absence from the CGI series coupled with his similar size and shape to Daisy are all points against his favor. However, I would be surprised if support for Boco did not skyrocket following a presumed announcement of Stepney, given that he would be one of the few classic series characters still missing from the line at that point. This is one announcement that I don't think we will see anytime soon, but one that we should certainly keep in mind.

Of course, I'm sure we will see more recolored wagons. I'd love to see the CGI Troublesome Trucks given some love, preferably replacing the current Troublesome Truck #1 and#2. Red express coaches would be a personal wish, but I doubt that Bachmann is in any hurry to reintroduce them to be honest. As far as new tooling go, I would certainly like to see the China Clay wagons in HO scale. And as much as I hate to say it, I'm sure it is only a matter of time before we see Nia and Rebecca introduced into the line. Given that they have been fully integrated into the main cast it is almost odd that Bachmann has not been compelled by Mattel to produce them, and their absence from the HO line (at the least! N scale Nia and Rebecca, anyone?) is notable and will surely be addressed in the (near?) future.

Narrow Gauge
Sir Handel- Given that Peter Sam is the first engine in the line to not feature a red-orange color scheme, I think that Sir Handel would further provide a nice contrast to the red palette that has dominated the engines of the NG line. He's obviously inevitable, however, I don't think that we'll see him announced until next February at the earliest.

Brown Brakevan- I'm shocked that Bachmann didn't announce this alongside the red and blue vans, but it's obviously on the horizon and it's only a matter of time until we see this recolor added to line.

Aside from these two choices, I think Bachmann has covered their bases in NG for the most part. We will surely see Duncan at some point, and I would surprised if one of the CGI NG engines wasn't in the works for an eventual release, but honestly I can't say what they're going to do next with this line. Everyone seems content with what is on the horizon and there are no standout requests that I have seen, unlike HO scale where Stepney is clearly a standout choice.

Large Scale
Edward- I would love nothing more than to see Edward represented in the range. Given that Bachmann has shown revived interest in the LS line, I think that engines such as Edward are more likely now than before. However, I am still unsure if Bachmann would take a risk on a choice like Edward. He would need a completely new chassis, and given that every engine in the LS line is a different color, his blue might make him too similar to Thomas in the eyes of a casual consumer. The number of times I have heard Edward mistaken for Thomas despite their obvious differences are too many to count.

Henrietta- Toby's coach is conspicuously missing from the LS line, and I think Bachmann is missing out by not including her. Plus, the inclusion of a face would make her more desirable to consumers. A fairly simple tooling that should certainly be considered.

As for my thoughts on the LS range as a whole, I was speaking with Chaz a few days ago when he pointed out to me that Diesel's listed price was less than Thomas and Percy. This raises some red flags because Diesel's chassis is fairly complicated, and Chaz expressed his concern that Bachmann might take the easy way out and give Diesel a molded chassis/details in the same vein as the Lionel Diesel, in which case demand for him would plummet immensely. This theory is supported by the fact that he was discontinued and then reintroduced, which might be a sign that they originally planned to design a more accurate chassis and then determined that it would be more cost-effective to go the route of the molded chassis and details. I pray that Bachmann does not do this, as I fear that if they did it might be the final nail in the coffin for the Large Scale Thomas line. I personally think that would be upsetting, and as an owner of the LS James, I can say that the detail and accuracy of the LS models is unparalleled and for the line to be abandoned by Bachmann due to one poor business decision would be a real shame.

N Scale
Henry- We've seen that an N scale Henry is feasible given his appearance in the Tomix line, and I think that he would be a good choice for Bachmann to test the waters of producing larger engines for their line.

Toby- A simple tooling of a timeless character. An obvious choice if they wanted to play it safe while continuing to expand the range.

Henrietta- A no-brainer, one-two punch if they announced Toby.

N Scale Christmas set- I would go absolutely nuts for an N scale Thomas with a snowplough. Please, Bachmann. Please.

As for other N scale rolling stock, I'm sure we'll see the exact same rolling stock available in HO scaled down to N scale announced over time. I realized this when I saw that they opted to release the water tanker, an odd choice but one that makes sense when considered in terms of the other ranges. I'm eager to see where the N scale line goes, but for now I am reserving any true speculation as it is still so early on in the game and the line isn't even released yet.

So that's where I'm at currently with Bachmann's Thomas range as a whole. Again, like I said, these choices/predictions are not all necessarily ones that I am expecting to see in July, as I am guessing that anything Bachmann has planned is already locked in and our input is probably not going to be super influential at this point (barring a recolor like Black James which clearly has a huge amount of support behind it and one that Bachmann needs to seriously consider). I'd love to hear your thoughts on all of this, and of course I am happy to elaborate on any of my thoughts for anyone who might be curious! 

Black James would be a huge hit. And if he had been released alongside LBSC Thomas I can guarantee that they would've been a 2-1 punch for most people, doubling sales for Bachmann because most people wouldn't buy one without the other.

https://twitter.com/bachmanntrains/status/1227250326421561345?s=20

Bachmann, whoever is in charge of your twitter account deserves a raise xD

I wasn't really bothered by the Busy Bee James until I saw your face edits, cheeky. What a missed opportunity! I would really love to see Bachmann reconsider the face (as we have seen them do in the past with the likes of Paxton) and consider one of the options presented. It would certainly make the model more enticing and stand out in more way than one.

Uhhh, guys?

https://twitter.com/traindude67/status/1226632837320847361?s=20

Well, here are my thoughts:

LBSC Thomas is genius and I will be getting one as a collector's item. Busy Bee James is weird and I really don't know how to feel about it. The new set seems unnecessary but eh. And the NG brake vans were inevitable. N scale James and the tankers are lovely. And Large Scale Diesel is amazing! What a comeback by Bachmann. Very pleased about these.

Bachmann stated on Twitter that the announcements will most likely be at the Toy Fair from February 22-25.

Don't know if anyone had proposed this, but what if we get an N scale Bertie or Harold, which would then constitute a deluxe set being made? (I'm omitting a figure of Sir Topham Hatt due to choking hazards).

I love this. Seeing people make improvements to the Bachmann Thomas shell itself rather than just small modifications is such a daunting task but such a rewarding one. Good luck to you! It's looking great so far.