To prevent rushing, one should not be able to attack within the first hour of each game.
Also, a new leader power can be the thunderstorm. Lightning comes down from the sky every half second and completely destroys anything within thirty feet.
Spartans should be able to jack Spirit of Fire when playing against other UNSC players.
Flood should be able to infect bases, turning them into giant graveminds with tentacles that can destroy anything.
Finally, to make the game perfectly fair for everyone, units must deal as much damage to themselves as they deal to their enemies. When a player uses a leader power, the other team gets to use that same leader power for free. If one player is getting resources faster than the other player, then he has to share.
“I'm not a doctor, I'm a medic."
"What's the difference?"
"Doctors cure people. Medics just make them more comfortable. While they die."