Billy Joe Saunders would be the obvious choice, plus he is a southpaw . His stamina needs improving though.
Canelo can fight: Brook, Thurman, Garcia. He can fight Andrade or Jermall Charlo at 154. He can fight David Lemiuex or Saunders or Jacobs. Canelo has plenty of options and ALL of the above fighters would fight Canelo at the drop of a hat for the exposure and the pay day. GGG can fight: Maaaybe Saunders Maaaaybe Jacobs Eubanks in 2018 lol Andy Lee. Meh, he's lost 3 times don't want to see it Quillin. Should be good until Quillin goes to sleep. He needs to rebuild first. 168 he can fight Degale, Zurdo Ramirez, Lucian Bute, Groves, Badou Jack. For the GGG list of potential fighters, expect loads of ducking. I can see Groves, Bute and Degale taking the fight. As for the middleweights they are afraid.
It'll be Cotto II or they can always drag Provodnikov up to 155lbs. There is also Bradley. Both will be willing to take the fight and put at a disadvantage. Pac and Mayweather won't entertain the idea of fighting above 150lbs and 152lbs respectively. And neither can Canelo below 155lbs.
Lemiux already said he is not doing catchweights. I doubt the other Middleweights are willing to drain themselves for him. At the full 160lbs limit I see Canelo having no problem getting a fight with the other MW.
Billy Joe Saunders has experience fighting hard for 12 rounds. Quite frankly, Saunders would cause all sorts of problems for Canelo in a unification fight at 160lbs. Small middleweight but still bigger than Canelo, a southpaw and is insanely hard to tag clean. He outboxes Canelo all night long and thats before we even find out if Canelo can handle real middleweight power. Saunders should be all over his management to set this one up.