I think the game will prepare to wind down after the lostbelts conclude. They are intent on dragging this out. No one expected there to be a lostbelt 5.5 main story and if there is 6 months between LB6.1 and LB6.2 each chapter could take a year to release barring anymore surprises like LB5.5.
If they do anything after the lostbelts resolve then it will be like Epic of Reminent, a short fun, but a self contained story to send FGO off on.
of course if they did do a part 3 then it would probably involve preventing a bunch of bad futures. Angel Notes Collab?