Sounds like you're already planning some cool improvements, and I'm looking forward to seeing them!
I don't want to change the tone of the game to be a tower defense, but I really wanted to be able to buy a unit spawner like in gauntlet. Or if not that; I got pretty tired of clicking after a half hour, so I just kinda wanted my coffin to spawn my cannon fodder units, while I clicked to place occasional "boss units".
Also, I think it would be fun if you got an upgrade (or something) when a coffin explodes as a mild comeback mechanic.
I trust you'll keep a sleek design and add some fun stuff!