The idea is a little audacious, it's to write a Bleach fanfic in the Soul King era. Creating a protagonist as old as Ichibei or at least someone who saw what the world was like before the Soul King split it into three.
The fanfic will start more than 1000 years before canon, eventually the MC, as he gets stronger, will meet the Soul King. You can take the freedom to choose the Soul King's personality.
Just like Ichibei was given the title "Monk Who Calls the Real Name", I thought the protagonist would be given a title like "Warrior who leaves wisdom and blinds all things". The idea was based on the Japanese feudal society of warrior monks and samurai.
The title is a bit philosophical, but the protagonist's powers are related to time and light In contrast to Ichibei's powers who I believe are to alter reality.
I'm not even going to bother naming skills because of how unlikely it is for someone to write this novel.
Continuing on, the Soul King would be sealed eventually. The protagonist would continue training to get stronger, because the grind never ends, interacting with Ichibei, the noble families, the formation of the Soul Society, training the first generation of captains and perhaps participating in the war against Yhwach.
After that, it would be the writer's choice, I would prefer the protagonist to realize his limits and seek new unorthodox ways to become stronger, perhaps even wanting to become stronger than the Soul King.
So he can search for Soul King fragments, create a Hogyoku with those fragments, or wait 900 years for Urahara and Aizen to create theirs.
This is the kind of story that could end before canon even starts.
Honestly, I think the best possible ending to this story is where he achieves godhood and changes the world. Transforming Rukongai into a true paradise for souls and perhaps even ending the suffering existence of hollows.