An obvious choice would have been Jack Dempsey, who also trained in catch wrestling and judo. Any others?
Don't believe Duran trained in other martial arts, but a tough mentality and good clinch fighting certainly wouldn't hurt as transferable skills.
Bareknuckle guys like Sullivan Jack Johnson too since he was renowned for his physical strength. I know this is controversial but could you imagine Mike Tyson with bare knuckles? The combination of hand speed and power would be unparalleled. Most guys would be afraid of taking him down and he could easily destroy them standing up. GGG and Beterbiev both had some wrestling back ground and obviously are outstanding punchers so they would do well.
Tyson is an obvious answer. Fun fact: In Art Davie's biography Is This Legal?, Davie's "martial artist wishlist" for UFC 1 included fighters such as Karelin, Hoost, (Tex) Cobb, and surprisingly, both Mitch Green and Mike Tyson. However Mike served his sentence in Indiana from 1992-1995, and UFC 1 happened in 1993, so the what-if matches like Tyson-Green II (III if you count the street fight), Tyson-Karelin and Tyson-Hoost never went into fruition.
Bud Crawford D1ck Tiger La Motta Fullmer Basilio Naz Chris Eubank Snr Marvelous Marv Haha Chuvalo Rahman Big George Dempsey Marciano Langford B-Hop Archie Moore Liston Duran Usyk Loma