How about a covenant Dominion. Covenant bases serve the same roles as the UNSC basses just covenant tech and purple and worts.
Anyways, one team has two basses and the other team only has one. The other team with the two basses is dominating the battlefield. The team with only one base gets an option to deploy a Scarab vehicle by doing this it will turn the game into a sudden death mode for the team that activated the scarab. If the other team can take down the scarab they win. If the scarab team take down another base then the scarab detonates.
Maybe even add Elites as playable characters?