If Sonic would ever be made into a movie it would most likely be under a third party company owned by Disney. They did well with Wreck it Ralph, and companies would trust them with managing videogame characters.
It probably wont ever happen, as the Sly Cooper movie that Sony was working on will not happen, but they say it will be coming in 2019. As with Sonic and Sly Cooper alike, if by some miracle it comes out of development hell, it will most likely not be any good. Not to mention they have been practically pissing money into god awful movies.