The downside of a draw-and-place tile system like Zombies! is that it cant easily provide the layering effect that ramps-up difficulty. By pre-placing the tiles we can ensure that they are in progressively harder/dangerous bands (though this dose require a bit more setup time). A placement system typically radiates outward from a starting node which means the players have few options initially. I'd at least start with a surface area of face up tiles (containing forests, ponds, grass, rivers etc) to give players some room to move around in right away. It might be possible to use difficulty layers with placement if their are some rules that trigger a transition, the simplest would be having 3 stacks of tiles which get drawn from, players start with the shallow stack and when its exhausted they move the the next and so on. Because the player needs to move to the location the tile is being placed their would not any functional difference between the two for play but their would be issues with confining the tiles to the playing area (Zombies! has this issue), it's a trade off between setup time (laying out tiles) and time spent during the game (picking a tile and placing it vs flipping a tile). Also with regard to the resource cards I proposed would be under the tiles, it would work equally well to have 3 shuffled decks of them with the player drawing from the one matching the type of tile which would indeed be simpler then laying them out before hand.
Also I'm going to revise workshops from my earlier post, rather then replacing the underlying tile workshops would be a smaller tile that is placed on top of a face-up tile. In addition they are no longer dead-ends only, rather some tiles have a 'room' space in their center, all such room tiles have the normal connection possibilities of a non-room tile and for movement purposes the room is ignored. Workshops can also be placed anywhere on the surface level.
Various monsters and 'AI' controlled creatures should lurk the game and they need to be controlled, in a normal Player-vs-Player game an opposing player is usually allowed to direct the monsters movement, that's not really a possibility in a cooperative game. So would simply give each monster a set speed and role a special directional die (Up, down, east, west, stationary x2) and move it along the chosen direction until it reaches an obstacle, trap or tunnel intersection. Most monsters would be limited to one instance but their would be a 'lair' tile for each monster which would re-spawn the monster on a particular role (think Pac-man). The players will thus be dealing with monsters continually once they the lair is found and the nastiest monster lairs are in the deeper layers with HFS being a slightly different one-time burst of monsters. Some event cards could generate a monster instance at the edge of the tunnel network with out a lair present, it would behave as normal but not re-spawn if killed. Different monsters have different strengths and weakness, such as trap avoidance, door opening or smashing, ranged attacks, movement stopping webs etc etc.