I think any possibility of a sequel, retcon, spin-off, etc. depends on how well his career goes after this. If we continue my comparisons with Hollywood, then situations when stars try to regain popularity by reviving their old proven hits are quite common. I think it's objectively possible to predict that the ending of Oshi no Ko will deal a major blow to his reputation as an author and given the weak synopsis and setting his new manga won't last long, but I don't want to try to predict the future. For example, the author of Spy x Family literally became a star thanks to a hit commercial work after years of unsuccessful attempts. But the fact that Aka himself already seems to be acknowledging the problems and trying to fix them after the fact is a good sign.