Aragon: really should have been in to do more justice to the battleground that was the Iberian peninsula. For me though they're not the most fun faction.
Burgundy: regionally they were quite important but I wouldn't really use one of the sparse faction slots for them.
Teutonic Knights: should be a fun faction, with lots of knights and heavy infantry in the baltic regions fighting pagans and orthodox russians. Definitely a candidate.
Serbia: my favourite! They were quite an important kingdom on the Balkans. One of the last Serbian kings almost agreed with the Pope to start another crusade against the Ottomans. And I love playing them in MTW:XL
Bulgaria: same of the former applies, but I like them less for some reason. Not sure why.
Kiev: I wouldn't use another faction slot for another Russian faction, but axing Novgorod in favour of Kiev does sound like a good idea.
Armenia: would be an interesting faction to play considering their powerful neighbours, all the moreso because they're christian. They're a nice faction in MTW:XL too.
Georgia: not sure.
Cumans: Orda Khan makes a good point considering steppe peoples. They probably would be the best choice, but I still voted Serbia because I like them better
Baghdad Caliphate: not sure.