There aren't many things that aren't possible idea-wise, but that also means there's a million things I want to add, already. Generally, if you want a chance at seeing something specific in the game, design the item model in MaD Lab to drastically increase the odds.
I absolutely love this game but I have a few tips for 3: 1 is maybe a multiplayer mode of some sort, 2 is possibly when you are on top of a fallen zombie/enemy you execute them with your weapon, 3 would be allies that have personalities and benefits.
If you were to make a sequel; Tweak the survival rate (Like everyone else says) and allow you to customize what your survivors use as a weapon so second floor snipers can use the hunting bow and the first floor melee uses the assault rifle.
I wish I'd seen paratroopers more often, but there should be a level in 8 where you can be in a trench holding off soldiers while the tanks take on the opposing tanks
"He has destroyed countless of us gummy bears, constantly breaks through our walls, has broken our champions jaw, and constantly attempts to escape the land, but the people in the stands love him, so we should continue placing him in the arena when we capture him and always keep the jawbreaker battling him no matter how bad he's feeling."
 "Tangerine Tycoon"){kind=icon}
 "Mutilate-a-Doll 2"){kind=icon}
 "Decision:Medieval"){kind=icon}
 "Coinbox Hero"){kind=icon}
 "Dead Zed 2"){kind=icon}
 "Learn to Fly 2"){kind=icon}
 "Endless War 7"){kind=icon}
 "Burrito Bison"){kind=icon}
 "Drunken Wrestlers"){kind=icon}
 "Realm of the Mad God"){kind=icon}
 "Once Upon A Life"){kind=icon}
There aren't many things that aren't possible idea-wise, but that also means there's a million things I want to add, already. Generally, if you want a chance at seeing something specific in the game, design the item model in MaD Lab to drastically increase the odds.