Nick has always been my favorite fighter, but I don't want him tarnishing his legacy. He would get schooled by pretty much all of the top-10 WWs and MWs because they're not dumb enough to sit and trade in the pocket with him and Nick is too stubborn to mix-in his BJJ.
BUT...
There is one fight that I've always wanted back 4 or 5 years ago. Nick vs. Matt Brown. I thought their career trajectories were on a collision course in ~2014 after Nick lost to GSP and Matt Brown was on a terror. Their styles/mentality/gas tanks were 100% going to result in true barn-burner...but Nick took a long break after GSP and Brown dropped back down the rankings after losing to Lawler in the Title eliminator.
As crazy as it sounds, I think they've both degraded along the same trajectory (well..Nick's a bit more do to the sauce..) that this fight could still be made and be a big draw. Obviously, I think it has to be a catch-weight fight because I doubt Nick could make 170 and Matt's not exactly a small WW who would probably enjoy only cutting down to ~178.
Outside of Brown, I don't think there are any notable/"sell-able" fighters that wouldn't exploit Nicks style of fighting and turn it into a boring decision win. And because of that, it's in Nick's best interest to stay retired. Don't pull a Chuck Liddell and try to jump in there with the "Big names" again only to get embarrassed.