Note: These aren't really rumors, they are more like cliches but they probably could still fit here... Alot of these go hand-in-hand too...
1: Max is going to turn evil and suddenly find a new inner strength that will enable him to wipe out the other people with a wave of his hand.
2: The twenty-third Generation Ash will still look and act exactly the same as he does now.
3: Team Rocket, Magma, and Aqua ALL have Main Bases that are equipped with conveniently located, easy-to-operate self-destruct-mechanisms.
4: Somewhere in the last Season of Pokemon, Ash will make a stupid decision, and everyone else will have to remind him of all they've been through to return him back to normal.
5: Likewise, there will be a Flashback Episode (There ALWAYS is in Animes) somewhere toward the climax of the series detailing how everyone got to where they are now. The following episode after that will involve a major plot twist that includes the Main Villain showing up and the situation getting ten times worse in the show.
6: The Main Villain will be an Ancient Evil that was sealed away by the greatest Trainers and Scientists of centuries past. However, once said Villain breaks free, a small group of fairly inexperienced Trainers will be able to destroy it once and for all.
7: When things really start falling apart, Jessie, James and Meowth will be the first to jump ship and turn good. Sadly, they'll be as useless to the main characters as they were to Giovanni.
8: The final battle with the Main Villain will take no less than 5 full episodes. This may even go as high as 10 if the Villain has multiple forms.
9: All of the Legendary Pokemon will band together and attempt to kill the Main Villain, and seem to get the upper hand. But, the Villain will then say that he was only using a third of his power and start using his full power, easily crushing the Legendaries as a result, and forcing Ash and Pikachu to fight him.
10: Somewhere in the second half of this 'Final Battle', Ash's Pikachu will undergo a startling transformation and become a new Legendary Pokemon. After this happens, the Main Villain will no longer be a challenge to Ash and Pikachu.
11: Even though the Main Villain was able to destroy the Legendary Pokemon, as well as 90% of the world, the main characters will somehow come up with a way to undo the damage and make it seem as if no 'Final Battle' had ever taken place.
12: All of the female characters are going to end up fighting each other over who gets to marry Ash.