I'd think maybe a three-way split could be good given crusade mechanics:
1) a Byzantine core, containing Constantinople/ greater Thrace and the Anatolian coast
2) an Anatolian kingdom, roughly corresponding to what a Seljuk crusade into Anatolia should take if it won (though, I know, historically they took basically ALL of Anatolia before being kicked back from the coast). Maybe we could call it Rum?
3) a Greece kingdom that would make an excellent target for Norman Sicily, could be Epirus/Thessalonica/whatever depending on how big greater Thrace is.
These, IMO, would be logical(-ish) crusade/jihad targets while hopefuly retaining some semblance of realism. Maybe make all three uncreatable for Byzantine Emperors, while allowing them to continue to create other kingdoms? (that is, no Emperor would create a despot to rule so close to home, but might for far-flung Egypt/Armenia/Sicily)