don't forget too add in House. semi-expensive units, on par with enclave if not stronger, but much weaker to pulse weapons that you can give as upgrades to your units. and since this is non-canon, can we add in the institute with the androids? start out around NCR skill, but end up near enclave or BOS skill. they would be cheap, but highly dependent on expensive upgrades throughout the game
I think adding too many factions would be... not good. A fix to the variety problem is too team up factions, so even though they may be called NCR you can also have Securitrons, and maybe Boomer Artillery Units, and a special B29 bomber.
Caesar's Legion could have Great Khan people, and White Legs, although those don't sound too great.
I think BoS should be West Coast, Midwest, and East Coast just to give them tons of variety.
And Enclave would just be Enclave, but they could have Enclave soldier, Sentry Bot, Mr. Gutsy, Enclave Officer, Fallout 2 Vertibird (used mainly for transport with low weaponry, Fallout 3 Vertibird (low transport, but has lasers and carpet bombing) Hellfire Soldier, Tesla Soldier, Deathclaw, and maybe a few others.
Maybe each side could have one mega unit. NCR would be a B29 bomber, Enclave would have Frank Horrigan, BoS would have Liberty Prime, and I guess the Legion would have Legate Lanius.