As far as I can understand, these are the current factions in game/in development/being considered

Catholic
English
French
Spanish
Aragonese
Portugeuse
HRE
Danish
Burgundian
Polish
Hungarian
Italian
Papal States
Sicilians

Orthodox
Kievians
Rus/Novgorod
Byzantium
Nicaeans

Islamic
Moors
Tiafa Kingdom
Egyptians
Turks

Pagan
Golden Horde

Looks good to me, but I would make a suggestion(s)...

Catholic
Scots - They were an actual faction during these times, and I think they should be represented as such. It may also be a little bit of pride in having Scottish blood for wanting them in.

Genoans or Milanese - The Italians were quite divided, and this isn't reflected in game. The were constantly fighting each other and everyone else.

Orthodox
Empire of Trebizond - The were actually 2 factions competing for the Imperial crown after the sack of Constantinople. This may curb the Byzantine Juggernuat completely though.

Kingdom of Cilicia - The "Byzantine" controlled Lesser Armenia in early wasn't even under thier control, and was nearly an autonomous. By High, they had sided with the Crusaders, something that isn't reflected and can't be done.

Islamic
Mali or Ghana - They were an actual empire, not just some rebel kingdom. Ghana was more or less pagan, true, but had very strong islamic influence and I think could thus qualify as a islamic faction. Mali on the other hand was a very influental and powerful Islamic kingdom, again not reflected.

Volga Bulgarians - If only to curb the wolrds slow slide into catholic christianity, The Volga-Bulgarians were quite the faction and hiers to the the fallen jewish kingdom of Khazar (Which would be interesting to play as, BTW).

Pagan
Cumans - They fought the Hungarian Empire, and lost, but nonetheless was a kingdom in its own right. If you don't mind tearing apart the viking campaign, you could use the FN_07 faction for these guys to reflect thier conversion to catholicism in the 13th century.

Lithuanians - These guys were involved in tons of things, more specifically all things involving Poland and the Tuetonic Order. They don't need a conversion event, becuase a Polish (?) king assumed the throne and converted the country.

I think this may be more work then what you are looking for, but I would do the work myself if you want.