Michael Spinks comes to mind.If he'd been a heavyweight in a different era ,his résumé could have looked very different. He arrived in the Holmes era and then had to deal with a heavyweight division that was becoming increasingly deep.
Hearns is another one .He moved thru multiple divisions whilst encountering excellent, tough opposition.A different time and era with more weight classes or weaker champions could maybe have seen Hearns winning an even larger title haul.
but of course, even in defeat, Hearns losses nothing, because at least he got the chance to fight some truly great & historic peers and in that sense it can't be taken from him, where as other fighters who get slotted into such great conversations fought no one and On Purpose!!!
Haggler. I would have loved to see him fight the middleweights from the 90s, as it would make so many great matchups. McCallum, Nunn, Jones, Hopkins, Kalambay, Toney, and the list goes on. Not saying he beats them all, but it definitely would give him better wins than the ones he got from the middleweights of the 80s.