If he doesn't get the trilogy with Alvarez I'd like to see him fight any of these guys: Jacobs, Jermall Charo or Munguia. I can see Alvarez fight Saunders next for the undisputed MW championship if he gets past Andrade (definitely no gimme fight there.)
Golovkin definitely won't fight Charlo, Jacobs or Munguia next. 50% Murata/Brant 20% BJS/Andrade 20% a built up fight vs. a random but solid guy like Sulecki, Falcao, Quigley soon 10% retirement
Murata would be a big money fight in Japan, then he should try Billy Joe or winner of Dever/Jacobs, then retire or move up and fight Groves for big drama show in the UK.