Fedor won't come to the UFC because his handlers won't let him. They won't do any sort of business with the UFC unless they can co-promote. No M-1 affiliation = no Fedor. Also, don't forget that Fabricio Werdum IS one of the best heavyweights out there, whether he's in the UFC or not. He was cut prematurely from the UFC I believe... after only one loss. Wtf? He's done nothing but beat ass since then.
That said, I think Fedor would get beat in the UFC too, and I don't think he needs to go there to prove anything. This guy beat a prime Cro-cop, a prime Nogueira (twice), a prime Mark Hunt, and a vertible who's who of "at that time" top fighters in the game. Everyone's pissed he's not in the UFC but at this point I do think it'd be detrimental to his career. The UFC is the #1 breeding ground for the evolved mixed martial artist, fighters who excel in every aspect and are more athletic and naturally gifted than Fedor ever was.
I think the simple fact of the matter is that time is finally catching up to him. He's still deadly and one of the best, but he's also aging. At this point he should do whatever he wants. If he wants to cement a legacy, he should fuck off from his handlers and do what he wants. If he wants to retire, he would go out as the first official "best ever". I just hope to god he doesn't let his management fuck him around and keep him fighting only once a year in order to make the cash.
What I REALLY want... is an instant rematch with Werdum. Werdum has said emphatically already that he's down with a rematch.