I doubt it would be Ikkaku based on his aim to reach Kenpachi, and now that Kenny has had a boost that chance is even slimmer.
Lets play with the idea that Unohana is now dead, that leaves 2 captain spots open, 8th and 4th. Since 4th is a healing division some one would have to match that criteria and I don't really know of hand who that would be. Closest thing I can think of is Urahara but that seems unlikely too.
The 8th though is more open. Maybe Hachigen?
Well, with the newly opened position of captain of the 8th squad I would argue that the next promoted captain will be love. He is the only captain level vizard who has not yet returned to the squads. Other than him I don't see anyone else taking a position as captain of the 8th division. Urahara has his own afairs to take care of and yoruichi and tessai follow him around.
Edit: yes love works. 4th though, they haven't shown anyone that fits. Might be a unknown shinigami. Well unknown for us.
I am of the idea that unohana will remain the captain of the 4th division. She took a critical hit but we have seen people take worst than that and survive. Even if her condition is critical there is no particular reason for her to not survive unless it is absolutely impossible for help to get to her for a decent while.
---------- Post added at 04:05 PM ---------- Previous post was at 04:02 PM ----------
Renji can have the opinion he wants on the matter but as far as we have seen ikkaku has never actually shown a level which would imply he is even vaguely qualified to be a captain. To boot his bankai is broken and by all intents and purposes useless. If his bankai really has lost most of his power then I don't even see what the point of it was, it would have never been meant for more than a single fight as anyone capable of matching the power of a VCs bankai would have been able to easily break it. If bankai are impossible to fix by anyone other than nimaiya then ikkaku's bankai was worthless to begin with.
Renji seems like a sure fire bet after his presumed power up. If we're replacing the 4th seat then it SHOULD be a healer, so Hachigen would make the most sense (temporarily) and because there is a war going on he doesn't necessarily have to have Bankai to qualify.
I don't know if it matters that they are replaced right now because if we include the royal guards, well they more than make up for the vacancies.
I don't think hachi is actually powerful enough to be a captain. His orihime-ish abilities would indeed make him an ideal healer but he still seems to lack power. He defeated barragan but unlike the other captains who fought espada he was not able to continue fighting after doing so while even soifon kept fighting after using her bankai twice. Renji with his powerup is likely to be relevant to some extent in the future however he still seems to be 8 years short of bankai training. Tessai would perhaps make an interesting captain of the 4th division although he was never technically a shinigami.
Love has been quiet over this arc, but I think the only reason he didn't return to SS is because Komamura is still leading his old division. It could also be that he hasn't forgiven SS for the past, like Hiyori, but I don't know. So he may return to the Gotei 13 now that they need replacements.
I'd say Hisagi would make an ideal candidate. He comes off as a responsible guy and a good leader, but it depends on if he has the raw potential to ever become Captain-class in terms of power. I think the same goes for Kira. Or went for Kira if he's dead. Oddly enough I can't really picture Rukia ever getting there, despite being a second tier character. As for Renji, I imagine he'll one day have all the power of a Captain, but I don't know if he has the other skills for it. It may just be an experience thing, but I mean, he has been in the Gotei 13 for a good few decades. I'd like to think he can still grow, though.
I don't think that's the case, especially with his apparently best friend Rose already back in his position. Love showed no particular grudge for SS, either. Probably because there wasn't a vacant spot.
Hisagi's demeanor is certainly superior to his VC peers. Kira, I'm not so sure, but Hisagi is a great candidate. He needs to achieve Bankai, though. I find it a bit odd that someone of his skill level is yet to unlock it, when Renji was able to.
I can't see Renji as a Captain. He is a good fighter and he will continue improving, but attitude-wise, he will never be a match to Byakuya or so it seems. I doubt he will ever surpass his Captain.
Renji got his Bankai broke by Byakuya in the SS arc so he hasn't really even been able to work on it correctly. I think he's getting a major power boost in the story now because it's going to be fixed.
To be honest my impression that is that in general the bare minimum to actually be a potential candidate for captain would be to have achieved the required power to master bankai. In that particular regard the only way for renji to be a potential captain candidate would be that kirio's food and the pool treatment somehow gave him an increase in reiatsu equivalent to 8 years worth of bankai training. Under that logic hisagi would not even be in the consideration even if he had achieved bankai over the past 2 years. Hisagi does seem to be a sort of sempai to the other VCs though, only matsumoto and perhaps nanao would be older than him.
Explain Hitsugaya then. I don't think he has mastered his Bankai yet he is a captain. Renji has had a broke Bankai since SS arc, so the increase in having it fixed along with the training he has had in the past and having currently should give him a decent boost in power. Thinking about it though I think he likes serving Byakuya or whatever.
Mastering means being capable of using it, not having perfected their bankai. Hitsugaya has increased his bankai skills in the time skip at least.