Ever heard of Red Dead Redemption Undead Nightmare? That game proves that it works in a GTA-type environment, and I'd imagine it works just as well from a top-down perspective
lol i didnt think of comparing it to that and i was about to play undead nightmare
GBH2 if you have time i think if you took and also made it into a zombie survival type game that would be awsome something you could play mp with your friends and stuff it would be really fun or even make gamemodes for it but i know its early in its development just giving some ideas