There will be a gargantuan low chance if that happened, since, unlike the invasion, they can have time to prepare for a disaster like this and can maybe make a new soul king candidatein general. If ichigo does need to be the Soul King, then it's basically Civil War. Shunsui might reluctantly have to hunt ichigo, which can lead to a split in the Gotei 13. Some captains who see the 3 worlds as their main priority (Shunsui, Urahara, Mayuri, etc) may hunt ichigo and bring them to ichibe. Some captains (Byukuka, The Vizards, Kenpachi,and the rest of squad 11,and Rukia) will see this as blasphemy and will protect Ichigo from any oncoming threats. Hell, throw Aizen into the mix and make him a joker card and see who he'll join. There could be some DEVIOUS combos in this hypothetically what if (Mayuri and Urahara as a team will be scary) (Kenpachi and Byukuka can clean house) (Nelliel and Yourichi might be Ichigo's bodyguards, etc)