I would love to see any kind of Steam Locomotive DLC. The modern trains are nice and all, but I will not be spending my money on any of the new DLC that have been released at this point. However, I would love to spend even up to $30 on a DLC if it had good old steam locomotives in it. (and it's well done, I don't want some glitchy bad looking DLC.) I think with all the new things you can do in the new TSW, would make a steam engine supper sweet; all the buttons you could use, the great sounds and graphics, as well as the physics, would make the old noisy iron-horses look great! Please consider adding some kind of steam engines to the game, I don't care if they are a passenger train or a freight train, I just want to sit in the cab of one of them old beauties.
I think a Union Pacific Big Boy or Challenger DLC from Chennyene, Wyoming to Denver, Colorado makes great sense and add features such as loading coal and emptying ash trays will make TSW interesting.
Steam seems universally liked though with it being seen on the mainline and the fact that few people are far from a steam railway you could say it is part of the modern railway scene. Visit one of the many heritage lines and there are plenty of young people and some of the steam loco developers for TS have been a lot younger than me and I was born a decade after the end of steam!
UP Big Boys and Challengers would be awesome. What about the 4-12-2s and FEFs? We also need ATSF with the 3751 and 2900 4-8-4s, Blue Goose Hudson, 2-10-2 "Santa Fes," or 2-10-4 Texas. N&W, C&O, Virginian, and others would be great, too. Also, NYC Hudsons and Niagaras, PRR steam, and SP need to be done.