For a wad like this, co-op support is really something that should be planned ahead, since retrofitting things which have reached this level of complexity could be really problematic. If it's still going to be done, the major things really are:
- 1. As mentioned, scripts which are meant to be puked should be marked as net safe.
2. All scripts should be run through to check for parts that would not work appropriately in a situation with multiple players. From there, you either rewrite them to work appropriately or create a branch with if (GameType() == GAME_NET_COOPERATIVE) or some other check.
3. The design of any problematic areas need to be reworked to avoid problems that will create impossible scenarioes (e.g. Player1 enters a room, the door seals behind him because it's a trap. Player2 cannot enter. Player1 dies and respawns. Both players are now trapped outside and the map cannot be completed).
If all of those were somehow appropriately taken care of, everything should largely work functionally and you would only have balance issues to contend with (placing enough ammo, more monsters etc...). However, barring how much work it may take, I'm not even sure co-op would go over great with Zen Dynamics. The maps are relatively small, closed quarters maps which would just become too cramped with even more players.
The short version: It could wind up being a lot of work with little to show for it. Sounds like a fun thing to try though.
