The question seems to be whether it is allowed to create what is effectively a competitor framework (on your own volition and without authorization from your employer, as in the case of MVIKotlin vs MVICore) under your own name (brand), potentially in your free time, but based on designs of an open-source framework managed/developed/maintained by your employer in employees' time.

And even if it is allowed, can your employer claim ownership on the basis that your work is derived from employer/company work.

However, I think this is unprecedented (I can't think of any similar cases regarding reclamation of IP), and we'd have to be actual lawyers to know the answer. I'm curious to see where this case will go.

You can, as long as what you are working on was already by management internally. Otherwise... you're right. Would only bring problems if/when someone is set on making your life hard.

Why would it be a problem if they were related to his day job? Keep in mind the libraries he was inspired by are open-source apache licensed code. Your argument only makes sense if you assume the code was related to proprietary tech and not just a piece of code you find on the internet.

That only works if he wrote them during work hours. If he didn't then why would the company own it? Anything you do off the clock is none of the company's business.