I guess there's no doubt that Manny could defeat GGG if he prove that he can beat Crawford.
Of course, Manny will not fight at the current weight of GGG, it will be a catch weight like they did with De La Hoya and GGG will be lighter but Manny will still be faster, that if GGG will agree knowing himself he will not have the same power. AFAIK, based on the articles i read, these 2 are friends but for the sake of money and entertainment, I guess they'll give it to the fans.
GGG vs Canelo
GGG vs Manny.
These are the fights I like to see.
Pacquiao vs. GGG could be made at 154 lbs. The biggest opponent that Pacquiao has faced was Antonio Margarito who weighed 150 lbs. At GGG's age getting down to 154 might be a struggle. I doubt that it will happen though. If Pacquiao beats Crawford there are still good opponents for him at welterweight such as Errol Spence Jr.
That's a good option as well, but if Manny will try to be the 9th division champion, then he needs to move up. Anyway, there's no pressure on him as according to experts, it will take years or centuries before anyone can reach an 8th division champion and Manny will remain as GOAT on that achievement for awhile.