Ukitake and his zanpakuto Mimihagi are next in line to replace the Soul King to try and return everything to the norm. It’s unsure to how he’ll do this but whichever way I’m sure that he’ll have to hold on to life in his weakened condition. Ukitake has never revealed his power before, thats why this is so important. Both to everyone of that world and to the Captains and Vice-Captains who can still battle.