Okay, first of all this will be a small mod released on ModDB (and maybe Steam Workshop) that is basically a new game mode.
What I plan on it doing is being able to create and use different classes.
How classes will work is that the host/admin will create a class that has it's own weapons and pickups (that replace the default ones in a Half-Life: Source, Half-Life 2 or co-operative map) from preexisting SWEPs (like the Quake Swep Packs or whatever) as well as assigning a player-model to said class. Clients will be able to choose out of the classes and play maps together with other players, think of it like a combination between Team Fortress, Samsara and Sven Co-Op.
There will be additional variables for servers such as: survival mode (lives for players), players damage players (why not?), NPCs explode on death, NPCs do twice damage, NPCs respawn, random enemy spawns, etc.
Another small idea I for a co-op based mod would be the ability for admins to change NPC spawns, say you wanna play HL:S with HL2 Fast Zombies instead of the regular Half-Life 1 zombies or replace Human Grunts with one of the many custom NPCs and so on.
The last thing I thought of for the mod would be custom physics systems, closer to different FPS games like Quake, Quake II, Duke Nukem 3D, Half-Life 1 and etc. As well as class specific "movement" such as one class that runs fast and another class would be at HL2's default walk speed.
One last thing, I'll edit my post and add new ideas if I think of any.
I know the idea seems really basic, but there isn't really any coop mods for Garry's Mod except for horror stuff, this is mostly intended to play actual FPS campaigns in G-Mod.