A lot of good points were definitely brought up in your post.
1. I would certainly buy a Stepney! Might even buy a second just to customize it!
2. Seeing how well Stepney merchandise does, I think this would be a hot item. Hornby did release the A1X Terriers with a Stepney livery that promptly sold out. I was lucky to grab one, but I'm still waiting on a model of Stepny w/ face.
3. Probably only Ryan due to the face that he's a popular character. I would buy both, but I'm a Stepney fanatic. My phone case has Stepney on it!

Since this is a big year for Thomas, I do very much hope that Bachmann makes some more announcements at the NMRA in July. I would like to see:

HO Scale
Stepney
Red Express Coach
Red Express Brake Coach
Celebration Annie/Clarabel (To go with your Celebration Thomas, of course)

Narrow Gauge HOon30
Sir Handel
Flatbed/conflat

N Scale
Spiteful Brakevan

Who I want to see for 2020:

HO:

-Stepney

-Red Express Coaches (you've made them before, I don't think it would be too hard to dampen the red paint you use for James into a maroon color)

-Beige branch line coaches (basically Annie and Clarabel but with no face. Very season 1 style.

HOon30 Narrow Gauge:

-Sir Handel

-Red and Blue Brake Van

N Scale:

-James

-Thomas, Annie, and Clarabel starter set

I'm team Stepney all the way. He's a very underrated character all around. And after Hornby discontinued the Thomas and Friends version of their Stepney, you'd be lucky to find one floating around on ebay for under $200.00. If Bachmann released their own Stepney, it would be a massive relief to everyone that they don't need to buy Hornby's dodgy new terrier (I bought two, trust me, very dodgy pickups and build) and Bachmann could get a leg up on Hornby.