My basic idea is based off the mission from Halo Wars where you had to assault the massive shield dome. The attackers spawn with ARs or BRs and have to break through the shield (which is also ringed by a fortress wall) and destroy the base. To make it more like a real siege, the defenders have only one life, but have access to power weapons that spawn inside the shield, and have their traits basic traits raised.
Attackers cannot pick up weapons, but can drive the non respawning Wraiths and Rocket/Gauss Hogs.
The shield around the base respawns quickly, reducing the effectiveness of wave attacks, bombardment or air assaults
The attackers win if they capture the base terminal inside the base, which would either spawn mines everywhere or drop a massive killball on the base, killing everyone inside, attackers and defenders.
The defenders win if they manage to prevent the attackers from destroying the base.
This would be amazing if Halo had a much larger player max. More fit for 16v16 than 8v8. I like the concept, maybe have it so defenders have a few lives instead of one, like 2-3. And the kill ball can be activated by Defenders ending the game with a tie or something