I think what would really sell (as I'm sure it's been mentioned several times already) and what I'd love to operate is a conventional / legacy / whatever 2-4-4-2 (or comparable) articulated little logging mallet like the one shown at the Northwest Railway Museum: https://www.trainmuseum.org/in.../steam-locomotive-11
Also. A small Geared locomotive that isn't $Rediculous.
The reason for this is because I see a few people in this scale / gauge who are into or in need of a small layout. These would maximize the play-ability for the gauge in the space allowed. Plus... Geared stuff is really cool. Where there's a few people after certain items, more are bound to follow if it's a good quality. I'd build a logging / mining railway specifically for these two.
Edit: And they have to be small enough to get around a curve tighter than O27