Play Quake Much? I see that where you got the water Graphics from. Quake 1 & 2 Maybe?
Brilliant and fun game 10/10. Really made my day after i was bored from watching the T.V .
I think you should make a Multiplayer 6-man survival round where you survive from hordes of zombies for as long as possible. And you should make it so you can build mounted MG's if you have enough points or mortar's if possible. The levels get harder, the longer you survive, and it increases in zombies every round. A bit like the style of Call of Duty Zombies.
I dont know if it will be hard or not, since im not a gamemaker myself, but i think what you need to add to this game is something you can search houses and find survivors. They act like teamates and follow you around and you can give comman orders to them. Also stuff like make your own fort or base and after a while you can make new town's wants you collected new survivors. Anyway is an amazing game. 10/10 Good luck with making No.3! :)