How about Defend the Generator(s)? which is similar to the HL2 campaign mission where you storm the Nexus, instead defend it. Could even try something similar to the Find the documents, where there are persons of interest you need to capture (but maybe not kill, otherwise you fail - food for thought). Maybe something like an insurgency mode, where there are reports of rebel weapons caches/rebel strongholds scattered across the map, and you need to find and destroy them. These are probably rebel orientated, but this is all up in the air :)

-Defeat the rebels
You have been ordered to sweep out a rebel camp near the citadel. We hear that the Gordon Freeman is heading their. If you have the chance, kill him. Defeat any people helping them. Take only a few to question them later on. If you can after that mission, turn them into troops. Now hustle hustle!

Hold Out the Fort:
You can spawn some entinties in the map (Ammo boxes, Suit- and Health Chargers, Forcefields) in the map and You have to hold out with Your Squad as long as possible, there would be some special events, like Antlion Guards, a Rebel Squad trying to knock Your Squad off Your "Fort", etc.
Once You were seperated from the fort, or You get killed, the game ends and the fort is lost.
This would be hard to implent, but maybe You find a special map for excactly this Minimode? Like Apartment Defence/ Mountain Defence or something like that, only with a Overwatch Base

With some work, I can turn almost any map into a fort defense map.

The three ideas sound good, I'll see how can I implement them.

a steath sniper mission, You play as the rebels and a group of highly trained snipers (as you are one of them) are being sent to wipe out a combine camp. They also said that breen might be there (50% chance of him spawning in) if breen is there. Note that he is in our top 10 wanted list. find him and kill him as soon as possible (famous words from capt. price. "only kill what you can kill")

another one could be a hostage rescue mission. "gos into roleplay mode" alright guys all we need to do is find gordan freeman and escort him back to the van. eliminate all hostiles on sight
LETS MOVE! (there will be atleat 50+ combines in the area so you need to be in cover at all times)

Multiple stage survival:
Sections of the map are blocked off. Zombies or rebels move to capture your generators or any other objective you can think of. If you lose your objective, a new section of the map opens and your objective moves to the newly opened section. As time goes on and objectives are lost, harder enemies appear.
Just an idea that's been put into a few games already. Seems a little fun in my head.

VIP
You have to escort breen to a certain point of the map, and if he dies you restart (not reload the map i mean)

that sounds imteresting

Why not another version of Player Vs Player?
My thoughts: One player (maybe more) controls a squad of combines, while another player (again, maybe more) controls a squad of rebels, and then going from there, one of two things happen. A) Combines have an objective to complete, or B) Plain ol' team last man standing.

attack the rebel lighthouse in dropships and tell dropships where to land and drop combine

These all things are hard to code, the only thing i would like
Is a sandbox or something like that.

Originally posted by Pocket Sand(paper):

Why not another version of Player Vs Player?
My thoughts: One player (maybe more) controls a squad of combines, while another player (again, maybe more) controls a squad of rebels, and then going from there, one of two things happen. A) Combines have an objective to complete, or B) Plain ol' team last man standing.

except this one.

Kill The Target:You and your Combine Squad have to manage to get to the place where the Target is. Then when the target is killed another target appears in an other place. Then it gets harder. Like:First the target haves 2 enemies. The next round it haves 4. The next,6. And it gets harder and harder until the game ends.

Or, Player Control: This time the Combine Squad is formed with Players. The players need to follow the Captain (player)'s orders. Like,Normal Combine Squad but with player-controlled combine.