Well, eventually they will be "persuaded" to join the Imperium as a full province, as opposed to a client/vassal.I don't want to influence power-hungry blues too much, especially playing as one, but OOC, I find the strategic value of ceylon as a vassal questionable since we get no trade from vassals and they can't really help us any more than we could help them if they were attacked except maybe to make timurid AI derp out a bit.
Actually can you mod in client states to appear a century earlier? They seem like a suitably Byzantine thing to do, especially if all the preexisting European states are gone by the time of reconquest.
Or maybe you can mod in the old provinces- Illyria, Hispania, Brittania, etc. as custom tags to be released?