I have finished the demo, and I have some interesting ideas to add build variety, these might be planned features or in the demo and I haven't seen them yet but I still want to write about them while I am at the peak of my hype for this game.
So far I have seen tracks, barricades, bombs/mines, and abilities. And I could for sure see builds be focused on all of these aspects. Id love to see some barricade synergy cards where either something special happens when they are attacked or just give all barricades on the board more health, along with some bomb synergy where the tile its placed on isn't usable for track development for a couple turns.
An upgrade system for cards would be cool, things like adding a damage booster to the track or upgrading it into a super track(the one that allows you to place the track over other tracks). would allow you to make your deck more powerful without bloating it.
Non-kamikaze trains you can deploy using cards would also be interesting! Whether they be mobile barricades that can move along a certain amount of track per turn or an artillery train that can siege enemy outposts and destroy tracks as an ability in a certain range. If you added an upgrade system for cards you could even allow the artillery to have more range or more artillery wagons to shoot at 2 things when its off cooldown.