Untill we know what limitations exist in M2TW (and until then all predictions are just speculation; we wont know how many factions we can have until we are able to test the boundries ingame) the only definate factions are those that we drew up for the RTW version - this means we have only 20 (+ rebels) 'official' factions.

Organised by broad cultural bounds:

Northern European:
England
France
Burgundy
Kalmar Union
Scotland
Saxony
Austria
Bavaria

Southern European:
Castile/Spain
Venice
Milan
Naples

Eastern European:
Hungary
Poland (in personal union with Lithuania)

Orthadox Christian:
Muscovy

Muslim:
Moors
Mamluks
Ottomans
Mongols

If, as is indicated by several sources, there is infact a limit of 30 (+ rebels) factions, that would give us a further 10 factions. No decisions have been made as of yet as to what these additional factions may be.