As for the Super Smash Bros, Mewtwo was pretty much in Brawl. I think all the coding is there, they just decided to remove him last minute. I'm not sure if Lucario will be returning, but I doubt Zoroark would ever work with illusion in SSB. As soon as you begin fighting, it could be very easy to tell who you are based of the animations of the special attacks, unless they put an extreme amount of code in just for illusion. In the end it just seems like too much work for too little gain.
I am not sure how many characters they will allow from each series this time either. With brawl it seemed like the limit was four from each game. That's the only reason I can think of them not letting Mewtwo in when he was so close. I don't see them getting rid of the trainer, but they might change his pokemon (mixed feelings there). That would leave one more spot for Pokemon if they don't increase the "limit."
I would rather have Mewtwo.
I am not sure what to use for an avatar, so I think I'll use a quark. They're small, so you may not see it.