Even though I often argue against the idea that Fedor is the greatest fighter of all time, I stand by the assertion I've made in recent threads. Even though Emelianenko isn't as spry as he was during his PRIDE heyday, I believe he could come back and be competitive in a different division. Fedor seemed to rely more and more on big haymaker punches later in his career and we didn't see his Sambo skills as often. I think if he rededicated himself to his career, got into a high level training camp (even if that meant coming to the states - something he probably would never do), and cut down to 205 (or even 185), he could still be competitive. True, he just turned 37, but I think he could be one of those guys who could fight 'til he's 40+ like Matyushenko.
Chuck Liddell. Don't laugh here - I honestly believe Chuck still had it when he retired. Chin? Gone, of course. That was his downfall. He looked absolutely incredible in his last fight against Rich Franklin. He was in great shape and was clearly winning the first round. Unfortunately, he did the same thing that proved to be his undoing. He rushes in headfirst, hands down, gets tapped on the chin and his lights get turned out. I believe he would've had a better chance winning his later fights (against Shogun for instance) if he'd relied on his wrestling and protected his chin at all costs. Chuck was a great wrestler, which is something you usually don't hear about since he was known primarily for his KO power. He would've needed to dramatically change his fighting style (something he clearly wasn't willing to do), but I believe he could've won some of his last fights, avoided brutal knockouts, and perhaps even be active today.
Bork. I know I'm going to get flamed for this one, but I've always been a Lesnar fanboy and it is Brocktober, after all. If he actually got his passion back for MMA, changed camps (and let people spar with him for real this time), he could be a fixture in the UFC's heavyweight division again. He wouldn't be in the title mix, but he'd probably mix wins and losses much like Cheik Kongo did. I think he'd be a decent gatekeeper and you know he'd be a PPV draw.
Gina Carano. She left arguably during her prime for a hollywood career. I can't blame her for that, but if she came back she could still be competitive in WMMA. She was one of the best and I think it wouldn't take much for her to get back to the top of the heap. She could never make 135 again (she did it once at the beginning of her career then started missing weight even at 140), so the UFC would have to create a 145 division. Even though Dana White says he's a long, long way from adding another women's division, we all know darn well that if Carano wanted to come back he'd be falling over himself to open up a women's 145 class.
So, who do you think could come back and still fight at a pro level?