Pokémon and fan theories go together like, well, Ash and Pikachu, and few things spawn better fan theories than time travel. In the series' fourth film, Pokémon 4Ever, Ash Ketchum's encounter with a young time traveler named Sam might've been the catalyst for one of the most iconic duos in pop culture. In one of the more convincing Pokémon theories, it's possible that Ash and Pikachu weren't brought together by a broken alarm clock, but by a scheme concocted by Professor Oak from when he was still a kid named Sam who'd been brought to the future by a Celebi.
The story of how Pikachu came to be Ash's starter Pokémon has always been strange. It's odd that Professor Oak would keep a Pikachu in reserve instead of more of the traditional Kanto starters. Oak also knows that the Pikachu is difficult to control, and it's not like Ash is a promising trainer, considering he overslept on the most important day of his life. It always felt like there had to be more to it when the Pokémon Professor allowed Ash to take Pikachu, despite all indications that it was a bad idea.
However, Pokémon 4Ever may contain the answers not only to why Professor Oak had a Pikachu in the first place but also to why he allowed Ash to take it. In the film, a kid named Sam, later revealed to be a young Samuel Oak, is taken 40 years into the future to the series' present day by Celebi. Here he encounters Ash and the two become fast friends as they work together to protect Celebi from the Iron-Masked Marauder, a viciously evil member of Team Rocket who is determined to capture Celebi in Poké Balls that seem to turn Pokémon evil.
Once Celebi's safe, it takes Sam and returns him to his own time. However, Celebi's time travel did not paradoxically alter the future. Earlier in the film, Professor Oak indicated that he remembered the events before they actually transpired in the film. At the start of the movie, Ash and company see a Suicune and immediately call Professor Oak, who confirms he's seen one before. The call ends before he can explain when and where but, considering a Suicune plays a large role in the film, it's near-certain that it’s the same Suicune.
During the film, Sam sketches Pikachu and Celebi as they sleep side by side and the movie ends with Professor Oak looking at the sketch once more, saying to himself that he remembers it like it was yesterday. It stands to reason that once Ash called him about seeing a Suicune, Oak knew that Ash was about to encounter the younger version of himself, which explains why he's suddenly looking at the sketch again. As time travel paradoxes go, it's clear that no one's future was changed, but that the events of the film were always what happened to Professor Oak as a kid.
Put simply, Professor Oak knew all along that Ash is the same Ash he met when he was taken to the future by Celebi. This puts his relationship with Ash into a whole new context and explains why the Professor has always treated Ash with a certain degree of favoritism. It'd also explain why he had a Pikachu waiting for Ash; because he knew the bond that the two of them would share. It's arguable that Professor Oak was being irresponsible by giving Ash a Pokémon that he clearly wasn't ready for, but perhaps Oak did it because he knew the person Ash would become.
The idea that Ash and Pikachu were brought together by a time travel paradox is fairly plausible. The only issue with this fan theory is that it hinges on the idea that the Pokémon films are canon, and there has never been much indication that they are. But it's easy enough to leave it up to interpretation whether they take place in the series' official canon. For the sake of a theory as compelling as this one, it's better to believe that Professor Oak knew what he was doing when he allowed a 10-year-old to travel the world with a grumpy mouse that likes to electrocute him.