Jones in his prime at 168 would have annihilated Joe Calzaghe. I don't think we really missed out by not getting to see that one.

The few that come to mind for me are:
If Hamed had hung around:
Hamed-Pacquiao
Hamed-Morales

And:
Trinidad-Mosley at 147 in 2000 (if Tito could have just hung at 147 for a little while longer)
Oscar-Quartey II
Whitaker-Norris at ~150 catchweight or 154
Mayweather-Tszyu