How would a lack of crusades affect the Roman Empire, assuming Manzikert and the civil war of the 1070's happen OTL? Would it either....
Have fallen earlier, due to lack of support to regain Asia Minor and no distraction of Muslim forces in Anatolia and the Levant
Or fallen later, due to the appaling damage inflicted by crusaders on the empire in the Balkans not happening?
Personally I think Byzantium would be able to reinvent itself as a powerful Balkan focused empire, retaining of course the pretence of being the "Roman Empire", and eventually evolving into a nation state named Romania. How about you?