I feel like what would be nice for the full game is another game mode that has multiple spawn points, that way you can get bigger swarms of heroes to fight, instead of just fighting 4 at a time.
You can then have the choice of funneling them in 1 single line, and hoping the heroes don't overwhelm your defenses when packed tight like that, or you can try making a path for each spawn point to deal with the heroes faster.
Ranged guards would be really nice as well, as well as maybe healing traps that heal your guards might be nice.
Maybe a secondary resource that's rare to get super powerful artifacts/traps, such as a teleporter that teleports each hero once for a round, so you can send heroes either back at the start, or maybe send them to an entirely new path. Maybe have a teleporter at the start of the dungeon that filters out specific types of heroes to a different trap area?