Yep, it's announced that those who purchased both the wii u and 3ds version of SSB will get Mewtwo DLC for free when it's released. If you don't buy both versions, I believe you can still get it but it'll be at a cost.
I know Sakurai is on vacation after working on this and doesn't plan on making anymore DLC anytime soon since DLC characters take A LOT of work. I wouldn't be surprised if within the next few years after things settled down that Sakurai would work on releasing some more DLC. If he does, I hope he focuses on bringing back veterans that seemed unnecessary to cut like Ice Climbers, Lucas, and even Snake if they get permission from Konami.