Of course they would be more upgrades to knights, wizards, and archers and new enemies... but what i want most is it to be longer, the first one took me like 2 days to beat... BUT it was fun!
The developers are still adding content to the first one, so don't get your hopes up for a sequel anytime soon. But since its mostly new enemies and maps, I agree that more towers and upgrades would definitely help. A melee tower that is radically different from footmen would be nice.