Gege’s probably gonna pull an End of Evangelion tbh. Since reincarnation is confirmed to be real in JJK, maybe Megumi activates the Merger and includes himself in it so he can be reunited with Tsumiki’s soul.
Yuji will use his soul splitting powers to free those from the Merger who want to retain their individuality, but some (like Megumi) will choose to say a part of the Merger. This would tie into the theme of only being able to save those who want to be saved.