INFINITE MODE
This mode would be infinite duhh but here's tje catch:
-Enemies get progressively stronger (around 5% every 10k) so it doesn't get easy
-This could maybe replace nightmare mode, since it's not hard to beat it at all
-And bosses like easter bunny, tesla, sasquach (or however it spelled) goliath and krampus could spawn (the chances of boss fights increasing the more you travel) and making the train slow down for the bosses to be able to keep the pace (this could make turrets and stuff like that more useful)
-Exclusive structures; these structures would would make infinite mode less repetitive (the reason they are exclusive from infinite mode is to make other modes not that crowded)