A great game for zombie lovers everywhere!
I suggest you make an online multiplayer version of Zombie Horde! I think fighting more zombies and less mutants would make the game more fun, in addition have a weapon carry limit, or the option to store extra weapons in a safe house. The flashlight feature is beautiful, and so is the control setup. It's comfortable and easy to master.
If you ever make an online multiplayer game like this, I suggest having character kits for players to choose (or unlock). Different starting locations, items that drop off of enemies (rarely), being able to barricade doorways, and starting with different item kits would make it a very popular online game.
I'm not sure I like the idea of in-game purchases. Instead, based on survivability and actions, you're awarded a certain amount of character customization points, that either let you edit your own personal starting packages or unlock new character skins. Items would run out in-game, and there would be no item spawn locations.
PVP in a zombie survival game. PVP should exist, but there should be a limit to it. I suggest that if a player kills a player, he should only earn half the earnable kill points. Also, he'll be immediately flagged as a PK'er, and for a five minute period other players may freely kill this player without point deductions or the risk of being flagged themselves. Known PK'ers will then have to be sneaky when killing other players, and have to hide out at the risk of being the target for a free-for-all hunt against him.
A weapon limit should exist, so being able to scavenge and drop items on the ground without them disappearing would be a fun idea. Players would then have the style of play where they have to collect goodies for a prolongued zombie siege in order to survive.
I think having a goal for the game would be a good idea. Maybe a random ending would appear at the end of each game. You might get a message to find a certain rooftop in order to be safely evacuated. Or perhaps the army appears and clears the streets. There should be a new ending each game, so players can't cheat by staying at the final stand locations. The final stand is the location where the game message tells you that you should go if you want to be rescued. Simply by being rescued you earn reward points which can be saved and spent on game options.
The game length might be hard to determine. It could be as low as a half an hour, up to a 24 hour period. Either way, I have an idea for latecomers. Latecomers could spawn at a highway full of crashed cars, or from a plane crash, or by boat. They arrive late, so they wouldn't start in a location that other players began in.
Zombies should spawn at realistic locations. The edge of the map, plane crashes, train wrecks, graveyards, doorways that players can't enter, etc.. Having a zombie spawn behind a player in a closed room is frustrating and takes away the realism to any game.
The game can be as basic as the other Zombie Horde games in terms of controls and weapon use. But adding the option to use doors and setting up barricades (if the door has a pile of lumber by it) would be cool. The CTRL key was used to enter vehicles, so why not make it a standard action button?
Oh yes. If this idea is ever used, please do one last request. Allow the use of empty hands. Some of us like zombie games where you start off with nothing but your basic unarmed attacks. They could be made so unarmed attacks do no damage, unless you spend points into an unarmed skill that makes your punches do light damage. And that's not just one-handed unarmed: I mean both hands unarmed. It's fun to be able to switch from being a super-soldier with tonnes of guns to playing as a civilian who is scared #@!*less and has zero weaponry. Zombie survival sometimes means outrunning and outwitting your enemy, and having weapons all the time takes away that special aspect of the game.
Anyways, this is just a suggestion and more of a wish list. I loved Zombie Horde, all three of them, but I can see something much bigger. Something a person like you who has the skills could possibly do.