If Ubisoft ever creates a 4th faction (as it seems evident that they want this to be a long running competitive game), my recommendation would be the Mamluks or Janissaries, for that Middle Eastern flavor. Its actually rather obvious, since they were roughly contemporaneous, they have a very unique visual flavor, and would provide the opportunity for more arid (and frankly, more colorful) stage design. I'll post some links to images for people to get an idea of what they would look like and wield.
Mamluks
https://upload.wikimedia.org/wikiped...ull-Armour.jpg
https://s-media-cache-ak0.pinimg.com...407f29535e.jpg
http://www.firstlegionltd.com/images...ukcatcover.jpg
http://www.firstlegionltd.com/images...ru066page1.jpg
https://s-media-cache-ak0.pinimg.com...8ece713e8f.jpg
Janissaries
http://pre12.deviantart.net/62f3/th/...kerembeyit.jpg
a) Nice job picking a single aspect of a group that spanned several centuries. The Mamluks were indeed slave soldiersat their origin, but they went on to be the founders of several dynasties. It's a dynamic that in fact has been repeated numerous times throughout history in various cultures. I could also nitpick some very troubling aspects of history concerning the existing factions: Christian knights on crusade throughout eastern Europe, Spain, Turkey and Jerusalem committed many atrocities. Samurai were notoriously brutal to the lower classes and to foreigners. The Vikings whole schtick was rape, loot, and pillage (as distinct from Norse, the people from which they hail.
b) I also like that you put words into my mouth, specifically 'celebrating'. This is a game about warriors from various cultures. Warriors as a rule did not do nice things, wherever they hailed from. If you get squeamish about "Mamluks" as opposed to Knights, Samurai, or Vikings (all four loosely used as titles for a warrior-class within their respective cultures) then I think there are other things going on for you personally about the Mamluks than ethically dubious history.
Again this is a game about warriors, and warriors kill eachother and do terrible things in general, no matter where or when they came from. I don't even really care about the specific name they give the faction, I just want to seem more variety in the cultures, weapons, and environments.