Welp, time to guess Oshi no Ko's endgame, I guess.
I believe Hoshino Aqua will become a God, in reward for his service to the people around him, and AI's legacy.
Japanese mythology is full of humans who became worshipped as Gods after their death.
https://en.wikipedia.org/wiki/Arahitogami
AI, at the least, has been heavily referenced to have become a God. IMO, this is why AI has not been reincarnated in this series: in the Season 2 opening, Fatale, the ending shows AI revealing herself in the audience as watching Aqua perform the Play Tokyo Blade. This is showcasing that AI is watching Ruby and Aqua, but as a God, rather than as a human. This also juxtaposes how demonic Mengo-san was drawing Hikaru in the last few chapters.
So, Aqua, in dying to protect Ruby, and the lives of so many others, to avenge AI, his Idol and Mother's death, will be given a choice:
Became a God and live in heaven with AI;
Or, be resurrected as Aqua, and live out the rest of his life as a mortal besides Ruby, Kana, Akane, Mem-Cho, Oka-san Miyako, and everybody else.
If this is correct, I'd expect Aqua to discuss his God-dom with AI, herself, before choosing to return to earth to protect Ruby.
Bonus points if Ruby and Aqua have a kid, and the child is Hikaru's reincarnation, but with no memories. Despite my immense love for Kana x Aqua (wonderful natural chemistry), I'm convinced AKanakane is real, and Kana and Akane will end up together. Extra bonus points if Aka and Mengo tease a Ruby x Kana x Akane x Aqua foursome.
This entire time, AI was a God watching over her children and protecting them.