have greece hold onto the lands it gained in asia minor after WW1. Assume that whatever WW2 analogue happens results in greece being occupied by/become part of the axis.
If the major partisan groups are communist and manage to self liberate, you could have them declare the Revolutionary Republic of Byzantium. If we then get them going their own way as yugoslavia did and having to build up their military to dissuade the USSR from preventing this, we have a powerfu Revolutionary Republic of Byzantium. Not sure how likely this is though