NG: Three more volumes of A Song of Ice and Fire wait to be
written. What shape do you expect them to take, and are their titles
finalized as yet?
GRRM: Yes, three more volumes remain. The series could almost be
considered as two linked trilogies, although I tend to think of it more
as one long story. The next book, A Dance With Dragons, will focus on
the return of Daenerys Targaryen to Westeros, and the conflicts that
creates.
https://www.infinityplus.co.uk/nonfiction/intgrrm.htm
Everything that will happen in Winds of Winter in the South was never meant to happen originally. Euron was never meant to be at Oldtown, but at Meereen, Faegon (Perkin Warbeck) vs Dany (Henry Tudor) would have been just another thorn for Dany in Westeros aside from her war with the Lannister regime.
But GRRM changed his plan, kept Dany in Meereen, married her to Hizdahr instead of a Greyjoy, breaking the Undying prophecy in the process, and had fAegon take the role of the original second act of the 1993 outline, that of a Targ claimant invading the realm just as the Lannisters won their war with the Starks while Euron was prevented from going to Dany because his captains forced him to invade the Reach, which was not a part of his plan.
Now the entire southern storyline has grown outta of his control, and he doesn't know how to resolve it. I honestly think he has no idea what he is doing with fAegon in the Stormlands, Euron in Oldtown, and Cersei in KL. Especially since George mentioned that he was struggling with several of Cersei's chapters in Winds. And who knows what the hell his original plan for Dorne was at the time Dany was meant to arrive first.
I think this is the reason why D&D took the ending for ADWD, Cersei's upcoming trial, Battle of Fire, Battle for Winterfell, and extended it to last until the very end of season 6, while Jaime and Euron arcs for that season consisted of material meant for AFFC, Kingsmoot, and Siege of Riverrun. It could be that GRRM really had nothing of note to tell D&D during their Santa Fe sitdown regarding this part of the story, so they used existing material to cover the Southern storylines while giving fAegon story to Dany.