Rejected Channels

Song Theory or Music Theory is too hard for copyright reasons. The music labels would have smacked us down for literally anything that we could have done. Or they would steal so much of any monetization that happened on the channel that it just wouldn't be cost-effective to make it work. Maybe there's a world where they can re-explore that on YouTube, or maybe they'll do it on TikTok, where music is more of a thing. For those who are interested and want to see what Music Theory could be, there is an account on TikTok called Song Theory who dives into music conspiracies relating to different types of music and news about them.
Book Theory is tricky because a lot of the stuff already covered on Film Theory already originates from books. It would also take too long between uploads to research and theorize about books. Unlike TV and films which are in the popular conscious and people are searching for, books are too specific and hard to make something that would make everyone's needs. There might be a viable solution there for that, but it's harder than people would give credit for.
Conspiracy Theory would most likely end up being taken down by YouTube.
Sports Theory could happen if they find the right partnerships.