1. More weapons, flamethrowers, grenade launchers (balanced by the fact that it'd be single shot (so reloading after every shot, think the launcher from Terminator 2. Your ammo capacity would also be fairly low, like 8 40mm grenades with 1 in the chamber at maximum, so you'd mostly use pistols), sticky/planted explosives with a remote control trigger (so you can set more traps aside from oxygen/propane tanks (if you can find them) and gas tanks.
2. Related but not quite the same, more pill types and different levels of first aid kits, the current first aid kits would be the more advanced ones and would remain rare but smaller first aid kits would be a little more common, something you'd find in cars, and some offices, would restore like 20 health (instead of 80) so if you use your big med pack to patch up someone who gets incapped, you can at least replace it with something to cover minor scrapes and bruises for yourself somewhere else in the level. The other pill types would be stimulants, which increase your speed, which would translate to walking speed, turning speed, and reloading speed, another pill type would be anti-anxiety pills, which temporarily steady your aim, like crouching without crounching.
3. More destructable/manipulatable environments/objects. I'd like to be able to pick up things and make a makeshift barricade or bar a door with a refridgerator, etc. Like when you know a horde is coming, it'd be nice to be able to hole up somewhere to defend yourself.
4. More characters to choose from. I like the 4 that we have don't get me wrong, the main people I play with are 2 other guys and 1 girl so the current characters are perfect for us. But some other characters common to horror movies would be nice. Would be nice to see other survivors that aren't playable but you see them out on the street or whatever (and maybe watch them get ripped apart by zombies, or try to save them but they won't follow you to the safehouse, they'll run off and get themselves killed.
5. More campaigns, we have a hospital and surrounding city, we have a rural area with a farmhouse, a rural turnpike that goes to the docks, and an airport, how about an army base, college campus, a resort, etc.