Which fighters would have done much better in other eras? (For style reasons, because of the color line, marketability, access to modern advantages in training, getting away from modern "advantages" like cocaine, etc.)
There are loads of guys who would have benifited enormously from modern weight divisions that fought in an era before their perfect division was born, Marshall, Tiger, Dillon, Papke, Wilde, Marciano all become terrifying in their respective "best" divisions.
I agree with most of these, but not necessarily Marciano. He mostly fought cruiserweight-sized fighters anyway. The heavyweight belt was a lot more valuable than even an undisputed cruiserweight title would be today, so Marciano probably benefited from his era. He came along at the absolute perfect time for a guy of his size and talents to become a legend. Marketable in that era, too. With that being said, Marciano would indeed be terrifying in the 1990's cruiserweight division.
Carlos Monzon would be a huge star today with all the tabloids and 24/7 news coverage following his every controversial move. Likewise, Ali in today's heavyweight scene (and, like Monzon, today's media) would be pretty damn close to a license to print money for whoever promoted his fights. The 130-140 pound incarnation of DLH conceivably could have been even bigger back in the late 60s-early 80s era having the likes of Ortiz, Laguna, Chacon, Limon, Mando Ramos, Arguello and Duran to fight with, especially since they used to have fights at the Forum and the Olympic Auditorium and televise almost everything back then. This would have been great for both his celebrity and his legacy. At 147-154 though, he'd be better off staying in the present, because from that era he would outbox Palomino and be able to outbox or KO Cuevas, but his chances against the likes of SRL, Hearns, and Benitez don't look so solid.
If Marciano came to the pro ranks with a gold medal he could be world champ in his second year as a pro and undisputed in the third. Assuming his eyes don't get to big for his stomach he could have laid down the most impressive reign in all of history, at any weight. Probably he would have ended up renewing his rivalry with Holyfield at HW, but if he took the Hagler route he could have ended his days as history's most dominant champ. This route is made all the more likely in that he would have moved up to CW.
Lew Tendler, the great lightweight southpaw, fought in the wrong era, because some fellow named Benny Leonard ruled the lightweight's for seven years. At any other time Lew Tendler probably would be a champioin of the lightweights...
If Bob Fitzsimmons did what he did in his own time, today, there would be no arguments whatsoever about who the greatest fighter ever was.
Do you think that he could have avoided gradually wearing down after a few years on top, as he did in real life?
Unquestionably. He'd be fighting less often on the way up and over shorter distances when he made it - with no juggernauts in his eye. I think that angle gets overplayed as far as it goes, anyway. His last performance wasn't his worst.
I think Marciano is the wrong answer. As a Marciano fan, I'm not sure if another era existed that benefit him more so than his own. At least not a more modernized, I'd think if he chopped down the bigger guys of Louis era people might be more impressed with him and his H2H abilities. At the same time he could endure more wear & tear from such fights, so it's hard to say. It would be impressive and horrifying to see, though. Whenever this type of question pops up I think of guys like Harry Willis or Jerry Quarry.