I propose you a new hero playable in escarmouches, blitz and terminus. It would be the didacte the forerunner unity because now he possesses the soldiers, crawling, the knight, and in unity heroic > eternal watchman.
Moreover, the story takes place on the ark. Forerunner Bases could come out of the earth by a forerunner elevator and in ultimate power commander, it would be recomposor.
The last thing in terminus, if we could choose the units which Attack us, but that would imply that the score would not be seen.
Idea : terminus challenge
Parias challenge which would be attacked by equips UNSC
UNSC challenge which would be attacked by equips Parias
The challenge of the survivor, the player chooses his equips: Parias or UNSC. But this time, the player is attacked by the floods
Other idea: challenge of the big enemies: forerunner vs the floods
To adapt the decor the depending on the attacking unit