I don't see why you'd
want to (re?)conquer western Europe as BYZ. Italy's not much of anything, and the near east is supposed to be absorbed by a empire ruling from Constantinople, but why wipe out the big three colonizers/military powers and kill off like 3/4ths of the game by 1461?
If you're going for World Conquest that's one thing, but there's a reason no Paradox game covers the time of the Roman Empire - total dominance is boring.
Edit: Actually, why should they get missions? England, France and Spain sure as hell aren't going to recognize Byzantium as their overlord immediately, so no free cores. Nobody is going to sit idly by and watch as BYZ swallows the entire civilized world, so no BB reduction...what are the missions supposed to give that you wouldn't get from just rampaging?