An upgrade that gives 2 more upgrade slots. Couldn't you just buy one and put it in an upgrade slot then buy another one and put it in one of those upgrade slots and etc?
They should make a multiplayer game where one person spawns monsters and the other player defends. The defending player wins if the monster spawning player runs out of memory and the monster spawning player wins if they get all the cubes.
There should be an achievement for: Killing an invisible sniper with full health in one shot, Get a fatal headshot on someone with full health that's out of view with a non-automatic weapon.