Frankokratia

All of Thessalonika's counties as a French Catholic - without holding (or serving a liege who holds) the Byzantine or Roman Empire titles. The wiki's cheeky route: in 1066, replace the Byzantine emperor with a custom French Catholic. You start controlling Thessalonika but the Byzantine title blocks it - so conquer into Hungary/Pecheneg lands, create the Empire of Carpathia, make it primary and DESTROY the Byzantine Empire title; it fires the moment the disqualifying title is gone. Or, with Northern Lords: finish Miklagarðaríki, then move your capital to France and convert to French culture and Catholicism for an instant follow-on unlock.