Yeah. I mean GSP is and always will be an all-time great of the sport, nothing but respect for him, etc. I guess he'd be a live dog in just about any fight. But he'd be starting as the massive underdog against Whitaker, Romero and Rockhold. Probably against Jacare and Weidman too, though who knows where those two are really at these days.
There's some very winnable fights in the bottom half of the MW rankings for him, but who really wants to see him fight Vitor or Machida or even Uriah Hall?
There are probably some interesting, relevant and more winnable fights for him back at 170. The problem is a fight with Woodley would probably be a snoozer. But GSP vs Lawler, RDA, Condit, Wonderboy? Those are all interesting. They don't fit in with the "GSP, middleweight champion" thing though...