Some kind of multiplayer/co-op mode would be nice. For example: each player starts with one chosen hero, and fight for who gathers more gold and survives. Other idea is that players could attack each other with the monsters (P1 clicks the group of monsters, P2 gets damaged?).
Minibosses could be also fun. 1 monster per floor with the unique ability, health system a'la Archeos (or different, for example dies when a hero levels up or something), a reward for killing it. I think the more mosters, the better.
I'd love an android version of the game!