Ooooh, interesting one... At the time that Fedor historically had a fight with Matt Lindland at 'BoDogFIGHT' (jeezus, some of these early 2000s MMA company names...), Randy Couture had just pulled a huge upset by taking the UFC Heavyweight championship from Tim Sylvia in dominant fashion, becoming the oldest champion in UFC history at 43.
If Fedor goes over to the UFC, its hard to see him not getting put immediately in a title fight. The other possibility is doing a Cro Cop rematch in the UFC, with the winner getting a title shot, which would butterfly away Gonzaga . Given that this was around the point where Cro Cop was declining (I still can't stand watching that Gonzaga head-kick KO loss), Fedor would probably dominantly win that match. Either way, whether immediately or whether we still get Couture vs Gonzaga as IOTL, I think Fedor annihilates Couture. Couture is older, slower, smaller, less powerful, and with arguably less experience against top-level heavyweights.
Assuming Brock Lesnar is still given a title shot, I would favour Fedor to beat Lesnar, although he's enormous and therefore has a chance to get a good shot off and KO Fedor. What I think is more likely is Fedor weaving under a Brock hook, stunning him with a casting punch over the outside angle, Brock shooting, getting reversed by a judo throw by Fedor, and getting TKO'd on the ground. Fedor becomes a huge star in the US, but the UFC overall probably suffers financially (compared to OTL) by not having Brock as Heavyweight champ. MMA may be seen with a bit more respect though for not having a professional wrestler as UFC HW champ.
I guess then Fedor fights Nogueira for a fourth time, beats him then fights Mir and Carwin. They're both interesting fights that could go either way (especially Carwin with that enormous power), but I think Fedor's great top-control grappling and ground-and-pound gets him the win. Where things get really dicey is when we get to JDS and Cain. JDS probably has the best chance of nuking Fedor with an uppercut (like he did to Werdum in his UFC debut), but stylistically Fedor is a tough fight for him. Cain is the one where we'll probably see a real war. Cain had great cardio and good fundamentals, but doesn't have the power and short-burst speed of Fedor. Should be an absolute classic. By this time Fedor is coming into the fight with four title defences, setting the standard before the heavyweight title defence record is even set OTL (Stipe Miocic, 3).
How do you think Cain vs Fedor goes? How do you think Fedor would match up with other heavyweights like Overeem and Miocic (especially given that age will start being a massive factor)?