But I don't know if there is really that much to fill the gap with for a whole game though.
Since Nomura is going to introduce an entirely new character, they could introduce he/she in that time period.....or they could also show Maleficent taking over RG, and they could make Braig play a role in that...they would show Isa's fall to darkness...since he obviously gave into it....they would show Leon and the gang...more Organization Somebodies...show some scenes of the Apprentice's experiments...Lea and Isa losing their hearts.....and they could show more of AtW's castle...and more backstory on some worlds that people hoped that would be in BBS but wasnt.
It would be a story more so based on Radiant Garden if you asked me...with a few other worlds in it...kinda like Days was (but better...and without "Missions").