I could see them having their final "match" in the gym. It's the only fight Ippo ever really cares about, it doesn't have to be for an actual belt. Just to see who's stronger.
Wouldn't be satisfying though, at least not for Miyata, he wanted to face Ippo as a force to be reckoned with. If he is broken, it brings more sorrow than closure.
Frankly, I'm fine with Ippo retiring, like others have stated, not everybody can be Ashita no Joe, and frankly, Ippo just lacks the drive to put his all into it. Like Takamura always said, he was more focused on making coach happy and proving his efforts were not in vain, it wasn't really a personal drive and his obsession of always looking at himself like the "challenger" stacked on top of that. He can probably become a sort of sparring partner regular to still be in the show and help mentor others.
The question now is whats going to be the focus? Aoki and Kimura being treated as jokes for so long kind of kills giving them that torch, even if Aoki starts pushing for realsies I have a hard time believing that I could ever take it seriously and although I loved Kimura during the Mashiba arc, that was oh so long ago and his mometum his dead. Itagaki I frankly despise as the arrogant little asshole that he is, though he is the one who Ippo can mentor the most by far, assuming that Ippo is still a constant presence in the gym.
Takamura is the other likely main focus from now on, guy is funny and has charisma yet brings weight to the table due to his feeling of responsability towards Kamogawa and his own physical issues, but he has too much of "unstoppable force" syndrome, I just can't see him loosing, I can see him getting very hard fought wins for sure, but not the other way around. He's bigger than life and I have a hard time thinking he could carry the series by himself for a long period of time, that the guy is pretty much a walking buffon in his downtime doesn't help, doesn't seem to have a family and anything going for him outside of his training and maybe the physiotherapist.
Miyata could be the other main angle to go for, but it would just feel strange to follow him constantly since he has always felt like "the target to beat". Mashiba and Sendou I'd say are also discarded from the getgo.
God knows Hajime no Ippo has had a too long period where it just stopped in its tracks keeping the status quo, so I'm personally excited to see where it goes from here.