Gusty Winds in Salmon of Doubt is an incarnation of Schroedinger's CatConsidering that it's a cat that's half alive (specifically, the front half), we don't know if the back half is still alive, and he passes a sign reading "GUSTY WINDS MAY EXIST"...
In Salmon of Doubt, a future version of Dirk is orchestrating the first few chapters.
Professor Chronitis is a Time Lord, and his time machine is his TARDIS.Yeah, that's right. This WMG was actually justified once. The end times are upon us.
All life in the universe is a result of a stable time loop.Life on Earth started because of an accident caused by another intelligent life form. Time travel is also possible in the Dirk Gently universe. What more evidence do you need?
Dave, from the first chapter of Salmon of Doubt, is a kangaroo.It's mentioned that the human race is extinct, and that kangaroos are being taught to speak. Perhaps the "heavily armed kangaroos" from the description brought about the death of the human race, and now rule the world?
Salmon of Doubt could have seen Dirk saving the Hitchhikers Guide cast at the end of Mostly Harmless.The story follows on quite closely from the end of the previous book, placing it at the end of the 1980s at a time seemingly before Mostly Harmless (or at least the bits set in what seems to be early 1990s Earth). The mysterious source paying Dirk could be trying to enlist his assistance in trying to defeat the Guide MK 2 and it's temporal reverse engineering (possibly the Campaign For Real Time from Life, the Universe and Everything). After all Dirk knows about both time travel and jumping into the dimension of the Norse Gods. Douglas Adams expressed an interest in 'fixing' the Downer Ending of Mostly Harmless, and a possible crossover between Hitchhikers and Dirk Gently, and setting the book in the same time frame as Mostly Harmless would have allowed the characters from Dirk's Earth to save the other Earths. However in his final TV interview he also noted that some of the ideas in Salmon of Doubt might instead be used for a Hitchhiker book, possibly indicating that he'd given up the crossover idea.