When do you think that's going to occur however and for which plot reasons?
I don't mean to interrupt, but I would like to give my own scenario for it...
I speculate that Xebec will come by the VERY END of this current arc... when Kaido/Big Mom gets defeated or on their end.... and what I think is Xebec will play the SAME ROLE in this war, as what Blackbeard did during Marineford.... he will come during the finale of the war, does something outrageous, shock the world not only with his return, but also for acquiring some powers... maybe the Poneglyphs copies and Robin, ORRR maybe an Ancient Weapon (which is possibly in this war, we may get a glimpse of one, because it was one of the targets Kaido and Big Mom wanted to do).... solidify his presence and where he stands on the pyramids, and basically make Luffy set his eyes on him for a future battle in another arc...
However, Xebec's full arc will come to fruition during the "2 emperors vs 2 emperors" arc.... as I expect this war will be between Strawhat Pirates + Red Hair Pirates vs Rocks New Pirates + BlackBeard Pirates...
And I simply predict that Shanks will LITERALLY take out Blackbeard, while Luffy will take out Xebec in that war... but I could see Xebec in some of the events defeating shanks, just for Oda setting up the "Luffy surpassed Shanks" moment... when he defeats Xebec...
Oh, and I believe Shanks came to Gorosei to report about Xebec himself... and if it's any close to be true... then Shanks and Xebec to be involved in same arc is very possible, the same way Shanks came to warn WB about Blackbeard before, and they got involved in same arc in Marineford...