Hajime no Ippo/WMG
The series will end with the third Sendou fight.
Basically a fight with Sendou marked the end of the first major arcs of the series (Rookie Kings and Japanese Championship), so going by Rule of Three the last fight of the series will be Ippo fighting Sendou in his first World Championship defence. Miyata and Volg will both be fought before Martinez. Specifically Miyata will be fought for the OPBF championship, before going to the world stage, then Volg will be fought as part of Ippo's rise to 1st seed (Volg will be world 4th or 5th rank or something). The series will end either just at the end of the Sendou fight as Ippo throws up his victory pose and his record is shown, or it will be really cruel and end at the opening bell or first punch.
Sendou will be killed by Ricardo Martinez.
In keeping with his inspiration, Sendou will at some point fight Ricardo Martinez and go the full twelve rounds against him, only to lose by decision and die from his injuries. This will add up to give Ippo another reason to defeat Martinez, and also perhaps let us get a deeper look at Ricardo's psyche.
Itagaki will leave the Kamogawa Gym, either temporarily or as a full on Face Heel Turn.
Itagaki can say as much as he wants about waiting for Ippo to vacate the Japanese Featherweight Title, but in truth, he wants to fight Ippo in a real match just as much as anyone else, and the only thing standing in his way is the fact that they're both from the same gym. The frustration of being so close to catching up to Ippo but never quite getting there is going to wear down his loyalty, possibly enough for him to quit the gym/join a new one, out of logistical necessity to make the match happen, or an underlying frustration about never excelling as much as he would want in the Kamogawa environment. Related to this would be the idea that...
Sawamura's new job as a trainer will come back in a future arc
And who better a student than a frustrated Itagaki?
The third-to-final fight will be Sendo vs. Ippo III, the penultimate fight will be Miyata vs. Martinez and the final fight will be Ippo vs. Miyata for the undisputed Featherweight title
Ippo and Sendo's fights have a beautiful simplicity to them -not very many fancy techniques, and in the end, it's whoever had the stronger punch and stamina. It would finally settle the question of who the ultimate infighter/brawler is between the two, and it would make for a neat leadup into the final fight of the series, with Sendo gracefully admitting that even though he's one of the baddest brawlers in the business, only Ippo could outfight him.
In the meantime, as Ippo's recovering, Miyata challenges Martinez, and gets his chance to demonstrate that his father's brand of boxing can not only succeed in the world but can actually TAKE it, achieving fame in finally unseating the champion.
The last fight will bookend the series - Ippo and Miyata finally settling their rivalry that began in the basement of the Kamogawa Gym, neatly tying up the story. Alternately...
The penultimate fight is Ippo vs. Miyata, and the final fight is Ippo vs. Martinez
The Sendo fight continues as usual, but Ippo and Miyata meet and settle their rivalry in a battle for the #1 seed, culminating in Miyata finally accepting that he's more than proven the worth of his father's boxing to the world, and gracefully accepts defeat. Ippo's question will finally get its answer in the fight with Martinez, discovering what it really means to be strong as he takes every single experience from his career and throws it at the seemingly-unstoppable Martinez. His experiences allow him to succeed where Date couldn't, and Martinez finally gets the challenge he's been looking for all his life. The series ends with the hardest fought KO victory Ippo has ever had, and Ippo finally becomes the fighter and champion everyone believes he is.