Some suggestions
Fix the lag, I can not stress this enough, even when i use the lowest graphic setting I'm getting lag when the enemies start to swarm.
Another option is to let people go back and play previous levels or allow us to recycle and reassign our points if we find ourselves getting stuck. There's a few times I've wished I could take back upgrades from one thing, and dump them into something else. Last, and endless mode would be nice to maximize the chances for players to get everything after they beat the main campaign. The game is fun, but stuff like lag and the difficulty balance need work.