I dunno. Most of the factions proposed here are, imho, just too close to the existing ones. There's no point in adding something "new" if it ends up just being a copy of an existing faction with unit compositions who may look slightly different, but get employed in the very same way.
That being said, I would love the idea of a customizable faction someone mentioned here. I know there was a mod which would allow you to mix'n'match troops with armour and weapons as you see fit, and having this in the game with a neat interface would be quite nice. Background-wise, the player could choose to design his own faction like a new kingdom, some knightly order, or a group of marauding bandits...
If it absolutely has to be a new premade faction... yes, something with camels in the desert might be nice. I do actually believe this faction would play quite similar to the existing ones, but at least their design would be sufficiently different from the others as to still make them interesting solely from a stylewise point of view.
I'd also agree that a bandit-faction might be very cool as well, simply because it would have to operate very different from a normal kingdoms.
Other than that, I'd have absolutely nothing against firearms if they get implemented correctly, meaning on a very basic level. Good damage, terrible accuracy, long reload times. And naturally they should be rather expensive, not to mention the rainy-weather-penalty would be worse than to crossbows.
But... yeah, I'd rather see siege warfare getting more options, like siege engines and catapults actually being used instead of just standing around as decoration.