I admit it would be a bit odd to play as a grunt, but if you know the lore of the Unggoy you would most likely think this game would be an awesome spinoff. Similar to Halo 3: ODST. The grunts had multiple rebellions, it would be cool to play as an unnaturally strong grunt or just a normal one. Maybe lead the rebellion? Sounds cool.
I think you’d have a much more familiar gameplay if you played as the Arbiter during the grunt rebellion. You fight alongside the Covenant (which was more limited back then, I think just the prophets and elites). You’d still have that Halo shooter feel, with classic enemies and lore, and still get to be involved in that piece of history.
Something similar to Gears of War 3’s Beast Mode would be amazing. Playing as Warden Eternal against an onslaught of Flood would be amazing.
A grunt rebellion game would be like firefight style tbh, since the strength of the Unggoy are their massive numbers and with number advantage, their aggressive nature.
If it had a normal campaign the maps would have to be crazily flooded with Grunts, it would look silly either way.
That would be a comical game to play. As everyone mentioned there are a lot of them to be able to do anything, so it would almost have to be done similar to Halo Wars.