Underworld hopefully but Elbaf imo is more likely:
1. Fullalead's event was covered. No need to stay there and Blackbeard likely will mobilize after the 4th RP anyway.
2. Kid was the 2nd most direct route after Law, so Elbaf should be next logically speaking
3. Urouge is the last Supernova without coverage and needs a space. Considering Elbaf exists in a place called "Warland", and considering Urouge is based on a monk, I can imagine him being involved in quelling the wartime effect of this region.
4. I also think the sky will inadvertently have a lot to do with Elbaf, first and foremost the God Elbaf being some sort of Sky Islander/Deity type. Again, maybe Urouge is involved.
Resolving this will result in the Elbaf faction as an ally.
Then the 4th poneglyph war with the 4 Yonko and potentially Marines at Lodestar or near it. Then Laugh Tale, then the Final War.