I don't think there will be a new team that they will face in National championship because Komadai is already the best team in the country, they have won both Summer and spring national tournaments which is why they are the best team to beat. But I'm sure that they will have new opponents in the Summer tournament and in koshien if ever they will win. The captains of the new team can be chosen between the second years and there are some second years who shone in Act 2 but wasn't shone in the past seasons like hiromi takatsu, but players who had experience in an official game like Kanemaru and toujou can be chosen but it is possible that a new captain can be chosen in the first years like Yui Kaoru, he is a leader and led japan in the little leauge world series. But Sawamura's rival is Raichi Todoroki. It may not be directly said but you can see that Raichi became the inspiration of Sawamura to become stronger and improve because of the Homerun he gave up during the first season. It was his goal to overcome that experience and he was able to struck out Raichi in the Finals of the fall. But I don't think they'll face each other again because they will face Ichidaisan third in the summer tournament after Ichidaisan third beat Yakushi (my predictions..) and as pitchers it is their role to lead the team. Sawamura became the ace because he led the team when Furuya was recovering. They leaad the team as much as the catcher.

Sawamura became ace because furuya pitching was flop, so i think its not fair and maybe on next tournament when the number will reset there will be epic fight for the ace number (when sawamura and furuya on prime)

Furuya was recovering, he wasn't able to overcome his challenges as the ace, not because he was a bad player but because it was too much for him to the point that it changed him and Kataoka needed to change his mindset and give him a fresh start. We will only know who really deserve the ace number on how Sawamura will play in their summer tournament.