This is what my phd is on
It started out with the theory that Datchery was Drood in disguise, come back to find out why his uncle tried to kill him or some such. Then John Forster (Dickens' friend) and some others said that Dickens had said the book was going to be about the murder of a nephew by an uncle, and Luke Fildes (the illustrator) said he would be strangled by the long black scarf, and that the final scenes would be in the condemned cell. But the theories of Drood surviving did not really die down entirely.

There are also a number of theories on the identity of Dathcery. If he is not Drood, he has been proposed to be Helena (because she has a history of dressing up as a boy), Neville, Grewgious, Tartar (who some people also claim is Drood in disguise), Bazzard (this is one of the most popular theories) and pretty much every other character who is not actually seen to talk to Datchery.

There was even Felix Aylmer, who claimed Jasper was a good guy (even those who believe Drood survive generally think Jasper tried to kill him), and that he was really acting so strangely because he was trying to avert an attempt on his nephew's lfie which had originated in some elaborate backstory in Egypt.

And Duffield thought Jasper was a Thug (a real Kali-cult ritual murderer, not the hooligan type).

Droodiana is fascinating. Think Jordan theories written by Edwardian litterati in the Times Literary Supplement and that sort of thing.