Seeing Greb would be interesting. Particularly Greb vs Walker or Greb vs Tunney. Some historians think Greb is the best fighter of all time and there is no footage of him, obvious choice for those reasons.
Sugar Ray Leonard vs Duran would be fascinating from ringside. The footage of that fight is kind of shit.
Barrera vs Morales 1 because its Barrera vs Morales 1.
something I think about a lot is what old fights would look like with modern day production value and camera angles (cameraman on ring apron, at distance, etc.). I think Leonard vs Duran would be so much more interesting if there were close up shots of those punches landing.