Honestly, Fedor will go down as the best MMA fighter ever, if he comes to the UFC or not.
However, in saying that, I think if he came to the UFC and took a 4 fight contract then these are the 4 fights I really want to see him take just to say that he beat everyone that would remotely "threaten" him:
Barnett (UFC better damn well sign him)
Couture (2 of the most respected fighters out there)
Sylvia (just have been wanting to see this)
If he beat all of them, then he wouldve beaten everyone in his Era worth mentioning in the HW division.
Again, Fedor is a legend of mma and it wont matter if he ever had another fight again, just that I believe those 4 fights would be a great way for him to go out on.