Like
PushDustIn
mentioned, Sakurai seems to be highlighting Lucas and Roy's veteran status while mostly ignoring Mewtwo's.
Mewtwo's tagline and Boxing Ring nickname are the only instances where it's brought up. Roy and Lucas, on the other hand, are pretty much always referenced as being veterans from Melee and Brawl (I'm recall Iwata even mentioning it on the Direct where Lucas was first unveiled).
On top of that, like I actually brought up on the day the New Content stream first aired, the segment after Roy's trailer where Sakurai talks about them being veterans, the wording he uses ("Of the many fighters from past versions, I hope fans are happy to have back these two that were particularly popular.") suggests that they were chosen out of the remaining Melee and Brawl veterans and could very well be the last veterans who will be added.
This whole thing is pretty much the main reason why some of us think that Wolf (and all other cut veterans for that matter) is not going to be coming back.