This is a wild idea of mine:
It's a type of capture the flag, but there's only 1 flag.
It also has a forked game-mode
1)
It starts in a big open space, Humans are the defenders, Aliens are the attackers.
The humans recieve chainguns, pulse rifles and luci's, while the aliens can choose to be Tyrants, Adv Marauders or Adv Dragoons.
The aliens must secure sheltered (thus safe from human weaponfire) spawns in order to advance quicker trough the battlefield.
Then they must breach trough the human defenses (humans themselves are the defense, they have no turrets nor tesla's in this type of game-mode!) and destroy/touch the building/flag in order to win.
2) (forked game-mode)
The same as above, BUT the humans do have turrets and tesla's.
Both types of defense are linked to their controlling unit (tesla computer and turret computer).
The controlling units are inmovable and are placed at a reasonably remote location on the human defense line. (This way, the Aliens have a shot at destroying the controlling units instead of just breaching the defenses brutely)
Winning:
Aliens can obviously win by capturing the target within the given time.
Humans can win by defending the target untill the timer runs out. (this can be set per map and is very important for the actual game)
Spawns:
At the beginning of the game, the aliens will recieve 2 sheltered spawns.
These spawns are the farthest spawns on the map so others need to be secured fast.
Humans have 3 spawns, they can't be destroyed, but they can't secure any other spawns.
Gameplay:
In the forked game-mode, Aliens can go for a brute breach which will take long. (attack, damage, humans repair a bit, aliens attack again,..)
Or they can attack the controlling units which is quick, but hard and dangerous.
The humans will generally use camping tactics that differ from location and goal.
They can go protect the controlling units so the turrets and tesla's protect the flag/target/building, or they can gather around the flag/.. for optimal defense.
They can man several points in their defense lines such as higher placed grounds, towers, covering walls, pits,...
Balace is a non-issue in my opinion, since you can fiddle around with the human-alien player ratio.. (for ex. to maintain a good balance, you'd need 1.5 times more human players then alien players, or twice as much, or ...)
That's it for now, more to come if others require it.
If you think it sucks, Please state why in a post.