Sisters of Battle! I could see them having a lot of unique features to them such as Acts of Faith which required a special resource pool. Perhaps one shared by everyone within the Adeptus Sororitas which fills by completing quests but requires the effort of all Sister players to fill. And they can be spent usually in small increments to use abilities. Such as Divine Retribution which would invoke a protective shield around you that inflicts damage back to your opponent should they harm you. Or the Emperors Touch which for a short time makes your damage pierce any type of armor. They would be specialized in using mostly flame based weapons which could distort vision. Celestians use Meltas and Multimeltas to rip through heavy armour and vehicles. Confessors would be the "Support" based class. Hell add the Fraeteris Militia in there why not. So many possibilities.
IG. "Only" human? Oh well, smaller model - better use of terrain, prone position, faster repositioning, better cammo coverage, better ambushing possibilities, more flexible armament, ridiculously powerful vehicles. Personal fragility similar to Eldar, yet lacking Eldar speed.
Maybe we should kick out the original 4 and use these 4 instead, look at the balance of the percentage it is quite impeccable fair across them all with some slight values being of....i kid i kid. SOB for my part, i would be playing them but they are not available sadly....for now.
The Guard. I would love to see 10 to 20 of our shots go into a Chaos Marine before he runs into the squad and tears us in half while cackling maniacly. And then the last guy standing manages to get a shot off into the visor slit and kill the bugger.
Adepta Sororitas would be a logical addition. As well as Adeptus Mechanicus. Dark Eldar seem to fit Not so sure about the rest. I'm afraid that too much variation beyond 4 factions we already have will turn the game into some kind of melting-pot-circus. The game will need HUGE territories and ridiculous amount of active players to cover everything. Don't forget about balancing the amount of players for each side 4-5 factions is more than enough
Dark Eldar!!! seriously how could people not want to play a race which has to cause people the worst pain imaginable simply to not have their souls devoured by Slaanesh?