Considering the way Forerunner technology morphs snugly into shape, and the Promethean Knight appears to be a vaguely human body wearing some kind of armor, it might be that instead of driving Forerunner Vehicles, you wear them.
You know. Step into some kind of metallic disc encircled by rings of floating forerunner-esque shapes, and they'd all re-assemble around your form, creating a body suit with weaponry attached. Or morphing into the center of a floating craft that other players can jump onto and ride.
Only problem is, Forerunner Armor on a Forge Map is essentially active camouflage. Kinda cheap.