Fedor-Barnett - earlier, in Pride, rather than the failed Affliction attempt
Jones-Rumble - Rumble dealt with everyone other than DC
GSP-Silva - the style matchup would have been very interesting
I know these threads always generate Fedor-Randy posts, but I believe Fedor would have handled Randy at any point during their careers. Terrible matchup for Randy, IMHO. Fedor was too fast, too strong...I would have bet that he either taps or TKO's Randy in a relatively uncompetitive matchup.