Stepney is a fan favourite classic character, and it would be wonderful to see him in the range. It would not only restore faith in the HO line, but also in the fanbase, and would definitely boost Bachmann's sales.
While characters like Rosie are welcome, it is obvious that they do not have as much demand as classic characters like Duck, Oliver, Donald/Douglas etc and cannot generate as much hype and money. Stepney is no doubt going to be a popular best seller if ever made by Bachmann. Hornby's version is among the rarest and most sought after Thomas models, so the demand is clearly there.

I really hope your post gets read by a Bachmann representative and that they will take all your points into consideration. Making Stepney would surely help them a lot!
Only a couple Bachmann Thomas locos really make me want to buy them, but Stepney would definitely be a welcome addition!