Actually, that rumor is already not a rumor since likely it will really happen. But first, GGG and Canelo should win in their respective matches against Murata and Bivol. It's a big money fight that even Canelo is not interested anymore in a trilogy, he will accept the offer. It's also a big opportunity for him to silence the doubts that he won the 1st and 2nd matches. Canelo will now be able to seal who's the winner by defeating GGG for the last time.
But GG also has the same goal, to win. Although he's already 40 years at the time of the fight, let's hope that he will still give a difficult fight to Canelo who is currently in his prime.
There's already a negotiation between Donaire and Inoue that will likely happen first.
I'd rather see Donaire having revenge win on Inoue before involving Casimero in the discussion.