One thing I do hope for if they play the "sequel" card is to have the Alolan champion be the PC from Sun/Moon, that is assuming you don't play as that same PC in this game.
When I mentioned that it could be both a sequel and a third version combined, one idea I could offer is that they make use of the fact that in Sun/Moon, you can enter an alternate version of Alola which I remember reading that that was there just to allow you to enter the world of the other game simply to change time around for specific things but what if this game takes place in that alternate version of Alola but after the events of Sun/Moon so for example, in Sun, you were able to catch Alolan Vulpix so in Ultra Moon, you're allowed to catch that same Alolan Vulpix because the worlds are effectively switched around. Maybe events that happened in the Alola of Sun/Moon had different outcomes in the alternate Alola that we'd see in Ultra Sun/Ultra Moon so in a sense, it is a different telling of the Sun/Moon story but you're only seeing the aftermath, if that makes any sense.
It's sort of like saying, in the Delta Episode of OR/AS, imagine if the meteor was sent to the alternate version of Hoenn, that version being the original of Ruby/Sapphire and then in a future game, you're taken to the alternate version of Hoenn that was depicted in OR/AS but after the events of the main game so you're sent to a Hoenn that had a meteor crash down upon it. It's not that we're re-visiting the Hoenn of Ruby/Sapphire, it's that we're visiting the Hoenn of Ruby/Sapphire as seen through the events of OR/AS, again, if that makes any sense.