A nation with CCR and/or Admin Efficiency can probably be played a number of ways. Different paths of conquest, form different nations, assume different religions, etc. Byzantium's ideas strongly encourage a singular path, but if you lean into that path it's quite potent. They synergise so well with Religious ideas that it becomes relatively easy to build a highly stable core empire before absolutism rolls around. And while I'd like some more end-mission modifiers as rewards which are in vogue, if I'm honest the main change I'd be interested in is getting rid of the Territory penalty to conversion, and that's more for general reasons than Byzantium specifically.
Oh, and a rework of religious zeal, but that's wandering off topic.