i love this game, but i'd like to suggest some things to add, to expand the replayability even more.
i'd love to see a multiplayer mode, or even endless mode with points and a worldwide leaderboard perhaps.
some more items? for example:
dice, which would give you a chance to skip your turn if you get an even number, or be forced to fire at yourself if you get an odd one. (might be unbalanced but i thought i'd maybe inspire you :D)
maybe even new enemies? new locations? more lore? that'd be awesome, got no ideas how to implement that though.
other than that this game is absolutely awesome and totally worth the price, it has plenty of replayability and an astonishing soundtrack