To be honest I think people just need more than 2 factions. ES tried to cover that by giving people the choices between leaders, but that only gave you a unique unit or two. The most succesful strategy games usually have more than two factions. Like Dawn of War, World In Conflict, Star Craft, End War, C&C. Company of Heroes got strategy game of the year, but even they had to add in 2 new factions.
If you notice people don't usually stay on the topic of the flood when it comes to another faction they either recommend the heretics or URF, but to face the facts those would be horrible factions as the heretics use the SAME equipment it just has *** smeared all over their vehicles and armor, and the Rebels are too poor to even afford vehicles. People see the flood as a good faction because they're different. In my opinion I think the flood would be the best new faction because they are different. I'd be pissed off when Robot Entertainment would just add another turret to a Scoprion and call this one a Triceratops. Or the heretics get a Wraith that shoots out two plasma mortars (creative? not really) and I'm not saying thats what RE has in mind either.
The sentinals would also not be a good new faction because they're just the same unit only bigger. Unless they added a transformer sentinal (yeah i just said it) that would be pretty funny actually. Seeing as there's little less in the halo verse (aside from the hover tree and battle toads) I see the flood as the last option for the most qualified new faction. So they can't stop a vehicle in the campaign boo hoo thats what touching up is for. They drove vehicles in halo 2 they can do it in halo wars if they want.
Chuck Norris had the cajones to join the Angels of Death....do you? www.theaod.enjin.com
Must be 1000+ years of age to join.