I've browsed some other co-op variants, and I've felt that they strayed too far from the simplicity of the core rules, so here is what I have come up with.
1. Draw an event card and resolve its effect.
2. Fight all zombies on your space.
3. Roll, move, and fight any zombies you encounter.
4a. If you end your turn in a cleared (that is, zombie-free) building/store/etc, draw from the item deck until you get an appropriate item. Take that item out, then reshuffle the deck. If no such item remains, you get nothing.
4b. If you end your turn on a tile with unconnected paths, draw tiles to complete them. If you draw a tile that will not fit, put it at the bottom of the tile deck and draw again from the top.
5. Roll for zombie hearing, and move zombies.
Tokens- I use heart, bullet, and guts tokens. You don't need guts tokens specifically, but you will need something like coins to replace them.
Heart tokens - when a zombie successfully attacks you, you lose 1 of these.
Bullet tokens - Like normal, you can spend these on a 1-to-1 exchange to add to your combat rolls.
Guts tokens - Spend these to reroll a combat roll. On a natural 6 (combat roll only) you gain another guts token to a max of 5. On a natural 1, you lose one of your guts tokens.
Combat - Whenever you share a space with zombies, you must fight them. Roll a six-sided die. On a 4, 5, or 6, you kill a zombie and take it as a trophy. You may then attempt to attack another zombie on your space, if there is one. If you roll 1, 2, or 3, on your combat roll, then you fail and your combat is over. You may not attempt to attack another zombie this turn. You may spend bullets and/or guts to modify your combat roll (as explained above).
Zombie hearing and movement - At the end of each round (once all players have moved and are finished), roll a six-sided die. Zombies within that many spaces of a survivor (orthogonally) hear them and move 3 spaces toward them. Count spaces for hearing the same as movement spaces. Zombies cannot hear through walls, so "noise" must travel through doors. Two zombies may occupy the same space. If a zombie reaches a survivor, then it attacks! Roll a six-sided die. On a 5 or 6, the survivor loses one heart token.
Un-surviving: If you run out of heart tokens, then you die and come back as a super zombie! You no longer draw event cards. On your turn, roll two dice and use the higher of the two as your movement roll. If you reach a survivor, you may attack! Roll a die, and on a 5 or 6, the survivor takes damage. You can spend a zombie trophy to reroll your attack. Super zombies can only be killed with a roll of 6. Also, a player can move away from a space he shares with a super zombie without killing it.
Event cards are a little different in these rules. You don't have a hand of cards. Rather, you draw one at the start of your turn and resolve it immediately. In general, you are the target of a card. Here are the specific modifications to the core deck:
* "Where Did Everybody Go?" - Move 5 spaces toward the game's starting position, fighting any zombies in your path. If you fail to defeat a zombie, you stop.
* "Bad Sense of Direction" - Return to town square. You do not fight any zombies along the way.
* "Slight Miscalculation" - As written, but you must play it on a building on your tile or not at all.
* "Butter Fingers" - Discard a weapon or two bullet tokens.
* "Zombie Master" - Place 5 zombies on any legal space on your tile (1 zombie per space).
* "Fear" - You may not move by any means during this turn.
* "I Don't Think They're Dead" - A zombie surprises you! Roll two dice. If either die result is a 3 or lower, lose a heart token. You may use guts and/or bullet tokens to modify these rolls. You do not gain a zombie if you succeed on this roll, and no zombie is added to the board if you fail.
* "Brain Cramp" - Roll a die. On a 3 or lower, you must forfeit your movement this turn.
* "Your Shoe's Untied" - Divide your movement roll this turn in half, rounded down, to a minimum of 1 space.
* "Just When You Thought It Couldn't Get Any Worse..." - Place a zombie on every legal space in a building on your tile not already occupied by a zombie. If there is no named building on your tile, this card has no effect.
* "We're Screwed!" - Place a zombie on each of the closest 10 legal spaces.
* "Hysterical Paralysis" - Skip your turn.
* "Claustrophobia" - You may not enter any building this turn. If you are in a building, you must use all movement to attempt to exit by the shortest route possible.
Victory: You must clear all zombies from the helipad tile, and all current survivors must stand on the helicopter space.
I like this co-op variant a lot! I was thinking also of having a grid of tiles on the table already out but face down, then adding POI (Point of interest) tokens on top of these. The object being that you are on a mission to save some or all of the POIs and get to the helicopter.
I also considered using the grid pattern and dice to determine where new zombies would spawn; borrowing the mechanics from Flash Point Fire Rescue.
I can incorporate your super zombie and a turned player goes on the hunt for the POI's.
I don't mind "fixing" the rules of a game if I really like the components of it.