I do think it'd be cool if they somehow incorporated Lucas's support/healing/defense role in the party when designing his moveset, but I think he's more or less okay as he is. And if leaving him unchanged makes it easier for them to get him into the game I'm all for it.
And while I am upset that he didn't get in, and miss him more than anything, I can definitely understand why other characters would get priority. Lucina and Dark Pit are a lot more relevant than Lucas, since their games came out more recently.
I don't really know about Dr. Mario but.
Both Awakening and Uprising were really popular, and I think I remember hearing that Awakening basically single-handedly saved the FE franchise. Also I like playing as Lucina so I really don't mind her being in.
That being said, I really do hope they bring Lucas (and Wolf) back. As already said above, while they're very heavily
based off pre-existing characters they have plenty of properties to distinguish themselves. And man I *love* Lucas's attacks. Playing as him is just so, so fun.
They've got lots of fans and a ton of support, and while I'm no game designer, I can't see it being extremely difficult to bring them back. If Mewtwo does well, as I expect he should, then it could definitely open the gates to more DLC like those two. (My only concern is how Lucas and his game seem to be somewhat controversial. I just hope that's not enough to rule against his favor. He was apparently supposed to be in
since melee, so hopefully that gives him some extra consideration.)