That right there, the Greninja point, is what a lot of people seem to brush off, even though that is a very big thing. Pokemon X & Y was just as unfinished as Smash 4, but Sakurai was handed Sugimori's design drawings and design documents alone and just went from there. This is why characters like Spring-Man, Rex & Pyra, and others shouldn't be discounted, because this stuff has happened before with characters that'd be considered "too new".
All that really matters to be honest is how the status of all franchises involved in the game are and in-house development of newer releases of games are for said franchises for these more newer characters.