Meaning for instance maybe Floyd Mayweather Jr vs Morales based on their performance vs Carlos Hernandez OR their performance vs Jesus Chavez.

That is just an example. I think some fantasy fights would be interesting if it was only based on performances vs a common opponent.

Like getting Casamoyor Vs Floyd mayweather based on their fights with Diego Corrales!

The only problem is that you have to consider the point at which each fighter fought the common opponent. Otherwise, you might say things like Danny Williams could beat Larry Holmes.