I think in the middleweight division a guy like Luke Rockhold would be more dangerous for GSP than Bisping
Rockhold is really tall, strong, and has good grappling. Plus he's a finisher.
I don't think GSP could consistently take and hold Rockhold down for 5 rounds and I think on the feet Rockhold would eventually pick GSP apart with kicks
Weidman may also be a bad match up for GSP because of his elite wrestling and KO power. GSP took lots and lots of strikes in his last few fights, and if he gets caught by Weidman it would be lights out
He chose to come back when Bisping won the title for a reason. Bisping was a gatekeeper for most of his career. Yes, he was also underrated by people for most of his career, but he also lost to guys like Wanderlei and Kennedy who are vastly inferior to GSP
And he also barely scraped by a very old, slow, and one dimensional Hendo in his last fight. So Bisping is still the favorite because of his size, cardio, boxing, and KO power, but he is very beatable IMO