Hi! To be completely honest I don't pay much attention to other theories. But I have one favorite one. The idea of Dazai being the book is interesting, although it contradicts a lot the canon. I like the idea of the eventual collapse of the BSD universes. This one was not written by myself, but I tried to find an explanation to this collapse. I also like the idea of Chuuya's experiments being supported by his own family. And the one where Atsushi was experimented on as well. Other than these I find the rest, completely impossible.
Those I mentioned , could be possible but the problem with almost every BSD theories is that people can't explain why does it happen, it just will. Which is why I enjoy SOOOO much finding a reason to EVERYTHING. I am passionate about writing and plotting.
It's so satisfying... I guess it's the reason I enjoy writing smart and ambiguous things.