Have you checked out Chronobound's theory on Roy and Mewtwo's removals? I'm not going to delve into it too deep, but it speculates that Roy and Mewtwo were cut late in development, as they had individual victory themes at one point, while Doc, Dixie, etc. were removed early.
However, all these characters, whether cut early or late, have data remaining on the disc. One theory I could buy into is that Starfy, along with other possible AT characters, were initially designed for playability, but cut before undergoing any sort of moveset development. In this scenario, they do not leave character data behind, but look as though they could function alongside the existing roster.
This is all deeply theoretical, of course.