Great game, just wish there was an auto-save feature. I accidentally shut my game down on level 20 and lost all progress. Besides that little problem games epic, 5/5.
Maybe for cash gain, some little basic "Mission" type thing. Like: Get X amount of grenade kills. Take X steps. Get X amount of gun kills. Fire X amount of shots. But they only work if other people are in the room. Or something like that. A simple basic way to get cash because getting it takes forever.