...great careers (ATG/HoF?), but came up against a future ATG/HoF (maybe in Prime)...and were never able live up to that potential...
Taylor comes to mind in terms of having a lot of abilities, but I actually doubt he would've been an ATG, with or without Chavez. Weight problems at 140, lack of size/power at 147 and too much talent around him. If Chavez never existed, well Taylor still has his issues making 140 and goes up to 147, beats Aaron Davis for the WBA title, but WW in short time had fighters like stablemate Pernell Whitaker, Felix Trinidad, Ike Quartey, Simon Brown...Cristano Espana always would've been a very difficult style matchup for Taylor. I feel Taylor always would've been one of those guys who was a terrific fighter for a fairly short period of time.
Abdullaev looked good as well, up until he got crushed by Cotto. Kid Diamond looked to be the real deal but got smashed by Casamayor.