If there's to be another faction in the Balkans - that should be Bulgaria. I'd like to see the time frame devided into 3 or 4 periods. Maybe they can have Serbia in the 1300's since that's when it rose to challange the Bulgarians and the Byzantines. Meanwhile Bulgaria was at its zenith in the 1200's defeating the Latins and Epirus, stretching all over the Balkans (yes, including Serbia) and the Bulgarian king Ivan Assen II was called king of Bulgarians and Romaioi.
The Bulgarian army could be a mixture of slavic infantry, some Byzantine influenced troops and a strong heavy and light cavalry (horse archers too) since the Bulgars were traditionally good horsemen coming from the steppes and also they often employed Pechenegs, Cumans and other steppe tribes as mercenaries.