Plane Wars is always empty, I feel the reason is that when you join in, the lobby is empty and there’s nothing to do. When you leave, somebody might join after and have the same experience. All you can do is fly in circles and wait, or leave.
I suggest balloons, in the same style as the other minigames utilizing them. Stationary, maybe bobbing, but they give you an opportunity to get points. Fill the sky with balloons that appear/disappear after set durations, and you’ll have a reason to play solo. They could give less points than player kills, but having them ensures that there’s a reason to wait for other players, and it means you can still earn units for participating.
Something to think about. A larger map would be cool, but Balloons as extra targets would give you not only something else to shoot at with no need to program AI, but also a secondary target to shoot during a dogfight. Maybe higher point value balloons in tight spaces like the house and igloo tunnel, to incentivize risky moves.