i don't think he's coming back, and if he does it's ridiculous...why play games, if you say your retiring then retire.
its also ridiculous because if he 100% can't easily beat the opponent he risks losing and tarnishing his legacy/record...when he retired he made the comment about still having his health and leaving on a high note and on top, where most don't get the chance to do it because its too late.
I'd be disappointing in seeing him return, he would be going back on his word and really doesn't need come back and prove anything...all the other ww's out there are nowhere close to matching what gsp accomplished, it's almost a pointless subject to debate.
the ufc didn't want him to fight kahbib for the belt because of vacating the mw belt, they wanted him to stick around, defend, gsp wants to win the belts for more less put the final piece to his legacy...a 3 division champion, sometimes our selfish ways and obsessions eventually get the best of us. sure if the ufc goes back on there word and grants gsp the fight with kahbib for the belt, no strings attached to defend, can vacate at any time then sure maybe he would come out of retirement but it would only be worth the risk if the opponent he was fighting is guaranteed to lose against him.
In others words my advice to gsp is, don't come out of retirement and fu*k everything up you did, stay out and be one of the very few fighters who left the sport on top, with an amazing legacy to leave behind.