Indeed, I would love this.
Imagine it, getting to take on the role of a Mighty Sangheili Zealot, brandishing his holy blade and roaring triumph as he brings righteous destruction to the pitiful humans who cower before his radiance!!
I would play the hell out of that game. A chance to destroy all humans? I could do that.
To be fair though it might be hard finding a way to balance humans with Covenant races, what with how many different races there are, Grunts, Jackals, Elites, Hunters, Brutes, Drones, Engineers, Prophets etc. While all we have on the human side are well, humans.
And we all know how pitiful and lowly those abominations are, amiright fellas?
I suppose they could balance it out by adding the Flood and Forerunner machines as other classes and maybe even add the Covenant Seperatists as a potential 'optional' ally for the human side... even though said Seperatists would be trecherous heretics.