It might be an idea to invent a new gameplay. I do not know whether it already exists or not, but here’s my suggestion:
Both teams have some kind of ‘boss’ model, which has func_explosive brushes with high health properties. You could compare it with a raid in World of Warcraft. The teams have to ‘kill’ the other team’s model by nading or shooting it. It might even be possible to create a mod with healthbars for the bosses in the player’s HUD.
As team you can defend your boss or attack the enemy’s.
Requirements:
- The boss should be out of sight, so sniping with 5 players at it is not possible.
- A team should not be able to kill their own boss.
Is this possible somehow and how would you realize it? I think it would be a very cool gameplay, and maybe we could make up something cool together. It is a bit CTF-style, but I hate running around with objectives. I’d rather kill the objective 
What do you think about it?