Epic Mickey 3 was originally intended to be the final part of the Epic Mickey trilogy.
Warren Spector was asked about creating a trilogy. He said that was what he had in mind. These are some of Warren's exact words: "I don't do anything that isn't extensible. And I will certainly feel that I have not done my job if we can't make other games in this world with these characters. In my head, I've got two more planned."
Warren Spector also says that just because someone plans a trilogy, doesn't mean it will happen in reality. Spector has also confirmed that he had an "idea" for the story of Epic Mickey 3. Spector has stated that the musical bits of Epic Mickey 2: The Power of Two were a "test bed". Depending on the reception, for the next game he would have been going crazy with the musical songs, with Epic Mickey 3 possibly being a full blown musical. Depending on the timing the next game could have seen a potential release on the Next Gen platforms.
On January 29th 2013, it was announced that Junction Point Studios was being closed by Disney. Disney, however, do still have the IP for Epic Mickey, but no plans have been announced, meaning the chances of a sequel are slim. As is the case with many cancelled games petitions have been made by fans, though none have persuaded Disney to select a new developer.
Epic Mickey: Power of Illusion can be considered as the final part of the trilogy by some, but 1° It's not from Warren Spector, and 2° It's majorly considered more as a spin-off.
A brief teaser was shown at the end of Epic Mickey 2 that shows all the Petes going to Ventureland with a fearful Gremlin Prescott trapped in Big Bad Pete's hand. Pete then says: "Look at them all, so happy. They ain't even going to know what hit'em!". This showed the possibility of the Petes being the villians of Epic Mickey 3. If you also thin out various areas in the Mad Doctor's Attic in the sequel, you will find costumes that Gremlin Gus and Oswald aren't suited for. This hints that the Attic has become the hideout of the Petes, since the area is connected to Ventureland and that is where the Petes go after the events of the sequel. However, nothing will happen or change if you follow them.