A theme with All Might is defying the odds and continuing onward, so I’m hoping he’s going to live to be an old age.
Old and gray, having lived long enough to see Izuku become No. 1 before his health issues finally get a little too bad. He’ll die peacefully in a hospital, with a smile on his face, and while it’s going to be a better death than what anyone thought Toshinori would end up receiving… it’s still going to impact those around him badly.
Alternatively, straying away from my ‘ideal All Might death,’ I can see him being severely injured by Tomura, which would ultimately result in a very painful and sad death. It would be a catalyst for Izuku to focus on the League of Villains.