Ok this is highly likely to be in the wrong section but here is a suggestion for a co-op mode that would be pretty easy to do.
BM: co-op
info: co-op based maps for the multiplayer mode joined by anyone but with a single team each map would have an over arching objective.
FIRst map concept
the barracks
discription: defend the surface guard barracks from all hell breaking loose.
team: BMFS (security guards)
objective: survive 20 waves of hell defending an armory on the surface while trying to break into a motor pool to escape.
map info this map would be hybrid of the armory map and the car park map from surface tension with players spawning in the armory with shotguns unlocked at the start. players must defend a few NPC scientists that re in the armory (Can not be harmed by the players for anti greifing), while also helping an NPC guard secure floors of the car park every few waves once they make it to the top a working vehicle is found and you can escape.
map layout: players spawn in a 2 story barracks. on the top floor is an armory, bed area (door access to armory and stairs, spawn room), bath rooms, and roof access. the first floor has an office area, restrooms and 4 entry points. out side there is a small “yard area” and one other buildign with large walls all the way around, a north and south road access via double gates which you can not use. the second building is an office building which you can not access at the start. to the east is the car park which you also can not access at the start do to a 30 foot concrete wall.
waves and gameplay:
wave one: spawn wave, this wave starts 30 seconds after the map is loaded the first wave is simply headcrabs and hound eyes teleporting in, from this point on waves will spawn 2 minutes from each other with exceptions of the 10th, 15th, and 20th.
wave two: doors are broken down from a building next to the barracks allowing access from it spawns zombies and headcrabs random teleportation continues.
wave three: more spawns from teh building though they are mostly zombies jumping from the second story (players can not access do to a barricade)
wave four: houndeyes come in from a gate on the main road (similar ot level gates they players must use in campaign)
wave five: armory access unlocked vorts start random teleportation more zombies from the nearby building. (armory is unlocked in parts when first opened only the shotgun spawn is “unlocked” with all others being blocked by a keypad which a scientist is trying to brute force do to the head of the guard being dead or something. to to explain why things are delayed)
wave six: wave five with more vorts and less headcrabs/zombies.
wave seven: right side of map gets a hole blown in it this is the access point to the car parks entry, from this spews zombies, a dead soldier with a grenade spawn is found on the road. (East side)
wave eight: more randomly teleporting vorts.
wave nine: zombies from the car park tunnel, hound eyes from the road entry, grenades unlocked in armory.
wave ten: wave is started by a player activating a radio in the barracks as soldiers are calling for survivor locations. wave starts with a group of hostile soldiers from the road entry, followed by some random teleport vorts and then ending with more soldiers.
wave eleven: soldiers repelling in, and comming from the car park, first level of the car park is activated has a few mp5 spawns.
wave twelve: satchel charges unlocked in armory, IFV attacks from main road.
wave thirteen: mortar strike and random teleportation.
wave fourteen: first alien grunts teleport in on the roof next to the barracks. vorts random teleport into the yard around the buildings.
wave fifteen: players are told to head to the car park and clear out the second floor, main hostiles are soldiers. RPG spawn in car park activated.
wave sixteen: mortar strike blows a few holes in the barracks opening it up, and breaking open a crossbow spawn in the armory but also destroying the shotgun spawn. two tanks spawn on this wave with support infantry from both road entry points.
wave seventeen: soldiers spawn in mass and siege from the road part way through the wave manta’s strafe the road killing anything on it, another will strafe the office building allowing access to the second floor and destroying part of its roof.
wave eighteen: xen military attack lots of alien grunts and vorts. a manta will act as a teleport in vehicle taking it down will stop the hostile spawns (only part that would require things outside of scripting, mapping, and voice work).
wave nineteen: top level of the car park is opened a few soldiers are their but the are not hostile with a radio broadcast coming in asking for help, main hostiles of this wave are zecu and a few xen soldiers is the easiest wave sense wave nine, soldiers state they are abandoning their post and will evacuate you guys if you help defend their osprey as it fuels on the roof.
wave twenty: the last wave the scientists from the armory flee to the landed osprey found on the last wave. trigger a fuel pump to defend them, simply prevent xen forces from blowing the pump to win. on this wave a helicopter will spawn near the end but will not be hostile. radio chatter from the osprey reveals that the operation has fallen apart everyone is on their own, and the copter is giving cover fire to anyone that they can. (Copter only stays in the area for 30 seconds).
this wave is brutal with the players having to guard a point that is away from most ammo spawns, and with large numbers of xen forces. your only hint on the osprey and fuel trucks health will be if they are on fire or not. and well this map does have a bad ending with the fuel and osprey exploding taking the hole roof with them (implied all players on the roof die and the map ends) but if the players make it through their reward is a “Evacuation” via osprey for everyone that “enters it” (E to use camera set to its door get to watch it take off before screen goes black with [sector A security team: status safe… for now])
armory unlock tally
shotguns: wave, 5 with armroy access (destroyed wave sixteen along with ammo spawns)
grenades: tunnels wave seven, armory wave ten
satchel charges: armory wave twelve
mp5: soldiers spawns all waves with soldiers (no armory access) has a few spawns in car park.
revolver: armory wave twelve
RPG: wave 15 car park in the back of a truck with infinite ammo crates. armory wave sixteen when a crate with them is blown open.(metal crate)
crossbow: access blown open wave 16.
snarks: hidden in a vent access as early as wave 15
mp5 grenades: infinite crate on roof with osprey, near it is a few 9mm ammo spawns.
note: teh bases of this idea is to be a single map that is co-op focused, I based this on one of the more complex ones I have seen in HL2 M before the servers that hosted it died.