About Khazars and Eastern Europe steppes in general.
Khazars is interesting, but scaling them into game will place it into a two-cities (Itil/Sarkel, probably Derbent and Tamatarkha), twenty-turns (650- ~860-900) civilization.
I think, in european steppes we have a problem similar to Central Asia Turks: we have a lot of potential civ's (Khazars, Volga Bulgars, Tatars, Golden Horde, Crimean Khanate) but no one can represent all of them, and respawns with different names looks..crude.
I'd vote for Volga Bulgars (later Kazan Khanate) at 700AD as less nomadic and most stable, indy Derbent in 600AD start, indy/barb/Turkic Sarkel with Judaism spawns around 750, autorazed after capture.
It's a sort of ahistorical: for the opinion of most historicians, Tatars aren't direct descendants of Bulgars, but we can just think that this civ represents them all, like Mandinka/Songhai.
Golden Horde represented by Mongol's conquests, Crimean Khanate - as now, by flipping Crimea to Ottomans (and probably encouraging Byzantium/Ottomans settling here), Avars-Cumans-..-Nogais - by barbs stacks.
So, Bulgars
Spawns at 700AD at Bolghar (1W from now Qazan tile).
Core - region near Kama-Volga up to Urals
Historical - southern Volga region (represent Astrakhan Khanate), eastern Sibir and northern Kazakhstan (Sibir Tatars) and some contested tiles between Moscow and core (Qasim). As an easter egg - some tiles in Bulgaria (Balkan one)
UP: Power of most northern emirate
Spreading of religions (not only yours but all) in your cities provides additional hammers (similar to chopping).
UU (in terms of 1.14, don't look at 1.15 yet): Trade Caravan, replacement of Scout. Can conduct minor trade missons.
UB: (same remark) Some replacement of a Castle, cheaper and with more defense
UHV:
1. (???) Embassy of Yiltawar Almysh/Ibn-Fadlan journey. Build a Castle, Blacksmith, <..> in your capital at 1000. (or something another production-related, to represent building of castles with Arabian help and early medieval Volga metallurgy).
2. Battle of Samara Bend. Don't lost any town to Mongols or barbs until 1400.
3. Great Tartary. Control Volga region, East Siberia, northern Black Sea coast, and Moscow in 1700.
Names: Bulgar peoples, Bulgar Khaganate, Bulgar Emirate (after adoption of Islam), Khazar Khaganate (with Judaism), Bulgar Kingdom (Christian), Great Bulgaria (Empire rank).
Modern: Bulgar Republic, Idel-Ural State (fascistic), Bulgar Socialist Republic (communistic).
After respawn (AI): Tatar peoples, Kazan Khanate, Empire of Tartary, Republic of Tatarstan, Idel-Ural State, Tatar-Bashkir Soviet Republic etc