I think a cool addition would be more permanent defenses for your base, like sentry guns and possibly MG mounts that cost a lot of resources to make. Or possibly whenever you upgrade a tower, there's a chance of the survivor getting a better gun? (I just feel that atm towers don't really do much. I play a co-op game w/ my friend and we have 7 towers that can only kill 20 out of 75 zombies in a herd. Though this may just be me, and towers are having range upgrades soon so...