I think Sukuna will become as strong as Truth from FMAB. It just lines up. After all, this is the only way Gege can give a happy ending to the gang and not kill Sukuna.
I believe it would be possible after Sukuna absorbs the merger’s powers and ascends into godhood as the God of Jujutsu allowing him to total domination over the universe. But he will also gain the power to judge binding vows. This allows to be able to manipulate reality and concepts. So he will basically be able to create or do anything at a cost of nothing because he judges that the binding vow is fair.
TLDR: Sukuna will become omnipresent and omnipotent. JJK will now no longer be considered a weak verse, and compared to other verses like Demon Slayer and Chainsaw Man.