Right before I clicked on your post, I immediately thought about the Railway and Minecarts mod! Loved that you already had that on the list, and I look forward to the progress that you make on it!
There’s really only two things I can think of, and hopefully they’re a little less strenuous than multiplayer… which, best of luck to you on that one! If you accomplish some form of multiplayer in Stonehearth before the developers do, you will be a king to many!
Anyway, the first idea is terrain generation. Really, I’d just say add your own flavor to it. Whatever you feel would be the coolest thing for the terrain, go for it! I suggest terrain generation because that seems to be something that a lot of people want to see, and would refresh the feel of Stonehearth, since it’s been a while from the last update.
The second idea would be to look into combat, make it more strategic and more RTS-ish. Again, throw in whatever you think would be the best! I’d love to see what you come up with!
Oh! And for much more less-involved mod, I’d suggest a clothing mod for Hearthlings. Allow the player to create a wide range of clothes with varying colors with the weaver. Could be interesting!
Anyway, those were just some quick little ideas. Best of luck to you man!