Here are some plausible ideas for spin-off games: Ancient human warrior Ancient Promethean 343 Guilty Spark or some other monitor (for some type of building game) Dr. Halsey (for some type of spy or intrigue story) A Forerunner Lifeworker (an A-life game)
A game where you play as the Gravemind might be pretty cool, if it was pulled off correctly. The way I see it, it would be played like a combination of the zombies gametype, and the old Gamecube game, Geist. For those who have not played it, you are a ghost, and you fly around in the ethereal dimension possessing people/items to try to get back to your body. I'd recommend looking it up in order to "get" this idea more.
I'm imagining a kind of hybrid FPS/RTS, where the Gravemind's consciousness can possess different flood units, with the ultimate goal being to infect the galaxy. For example, if there a lot of combat enemies in a group, the Gravemind's consciousness (you) can possess the mind of a flood combat form, and take them out easily. Likewise, against a bunch of unarmed civilians, you could possess a flood carrier form and easily convert them to more combat forms to use. Infection forms would be good against slow moving enemies, and etc, etc.
Obviously there would need to be some more flood types (I can't even remember what the flood pure forms were) in order to keep things from getting stale. Just some food for thought.