Quit trying to stretch the teams dang goals, they're already adding new factions as they go along anyways. Zengids weren't even a planned original faction and now they're in. (Well, post 1.0 anywho)
I love the Mongols as much as the next guy, probably even more so but look at the factions the BC team has already provided us, I'm not going to complain.
Plus, Kypchaks are playable, they're good old Steppe Nomads.