But just because it says 'New challenger/fighter' doesn't necessarily mean that the character has to be entirely new. I view the term 'new' in the sense of each character coming
into the game is new to smash 4. You could also look at it in the sense of the other veterans that have returned. On the website, it would say 'NEW!' next to each character on their banners as they were updated. I'm confident if it were a newcomer Sakurai would have said it was a newcomer, but it says new challenger.
If Sakurai includes the Mewtwo from the Genesect movie this time, then technically she'd be a newcomer.
All we can do is continue to hope anyhow.
Oh and btw,
'Looks like we're nearing the bottom of the stash of new challenger videos that we've made... Like always, we put a lot of effort into it, so I hope you'll enjoy watching it live.'
Sakurai says looks like we're nearing the bottom of the stash here. That means, they're not done yet, so even if it isn't Mewtwo there's still more to come.