If you were going to take a break from "serious" work to work on something just because it would be really interesting, what would you do? --Paul Graham, https://paulgraham.com/greatwork.html
I would start writing a new philosophy of language and science and religion -- a grand unified theory of meaning and interpretation. I would jump off from the "Revelation Epistemology" of my 20-year-old youth and the intention of reading Gadamer's Truth and Method and start writing on understanding and explaining meaning. And that would require a lot of reading until the theory would formulate.
Or I would start working on the grand unified theory of physics. This would require many years of basic studies, while at the same time trying to read and understand the work that has been done, and trying to find the edges of current knowledge. Physics hasn't made serious progress in 50 years, it is said. As a young man I thought I had the capability to do that, and it would be super fun and interesting just to dig into that for the next 20 years and see where it goes.
Or I would try to invent a programming language that could be spoken with the clarity of English and the precision of Rust, so that it could be used for voice interfaces that don't depend on AI to interpret what is requested, just to process it. It would be really interesting to try to get that language right. It would take learning everything about compilers and interpreters and the syntax and semantics of existing languages. It would take years of dedicated work and be a lot of fun.
Or I would try to master the art of music composition and work on composing music that I love and that others might love.
Those are the things that come to mind: The same few subjects that I am always and forever interested in and unable to choose between. What if I didn't chose between them, but I chose them and no others?
There is one more that is more meaningful than "interesting" per se: I would write my way through Scripture, working all the time on formulating a biblically-based systematic Christian and philosophical theology that would help people who are stumbling around in the liminal space between conservative and liberal, between fundamentalist and unbelief, who believe and ask for help for their unbelief. -- That is in fact the project that I have begun working on at Scripture Forum. I don't know if it qualifies as "interesting" in the same way as the above projects, but I find it deeply meaningful and attractive to devote decades to helping others like me to have a more solid place to stand than I have had.