The list so far rumoured, as written on the wikipedia entry on the subject (dunno where they got the Mayans from)
The English
The French
The Scottish
The Holy Roman Empire
The Danes
The Spanish
The Portuguese
The Milan Territories
The Venetians
The Papal States
The Sicilians
The Poles
The Russians
The People of Novgorod
The Hungarians
The Byzantine Empire
The Seljuk Turks, Ottoman Turks
The Egyptians
The Moors/Almohad Kaliphate
The Mongols
The Timurids
The Aztecs
The Mayans
Now, assuming the 23 above are set in stone, they should add-
24.Sweden (I personally prefer Norway, but it was a weaker power than Sweden during the later years of the campaign period- although Sweden was also in the Kalmar union for a good while)
25.Lithuania (Takes up space, rather large state, although it didn't have any power at the start of the "grand campaign)
26.Bohemia (Well known faction, relatively powerful throughout the era- though subject to the HRE)
27.Aragon (Would make playing in Iberia more fun- nullifies the historically inaccurate Spanish state)
28. Serbia (Padding out the east)
29. Bulgaria (See above)
30. Teutonic Knights (new dimension to the Baltic wars- possinly emerging after conquering specific regions with the HRE)
Of course, in an ideal world, the game would also feature
Brittany, Provence, Navarre, Gwynedd, Powys, Dehuaberth, a few large Irish kingdoms such as Leinster and Munster, Norway, Iceland, Brabant, Burgundy, Pomerania, Prussia, some finnic and livonian tribes, Croatia, Zeta, Genoa, Jersualem, Antioch, Sultanate of Rum, Latin Empire, Knights of Saint John, Pisa, Empire of Trebizond, More Russian principalities and Muslim emirates, other crusader states- everything and everyone basically
