We really need a grass pokemon, also with a Mega Evolution, so I'd love to see Sceptile. I'd like to see another Megaman Character.
In the ridiculous dreaming section...I wouldn't mind Seeing Bayonetta
I personally still think that Bayonetta would be a great addition. But the odds of seeing more third part characters enter the game is slowly getting lower... so Bayonetta and
*sniffle* Snake mightn't be the best contenders to get back in.
While the odds of another pokémon candidate getting in are probably pretty low with the addition of Mewtwo, I also think someone like Septile would be a great addition. Though that might be bias since he was always my favourite Gen 3 starter anyway.
Personally I'd love to see
Abe Lincoln Lana, Impa, Captain Toad or even Zael enter into the game.
I feel like each of the characters mentioned could have fairly unique movesets that offer new playstyles to the game.
Lana could use her spell book and magic barriers to create some interesting effects, maybe even jumping from one to the other to make her capable o travelling distances quickly. Her final smash could be the summoning gate, used to summon Dodongo as well as the others for a "Pokémon trainer-esque" final smash attack.
I feel like Impa would be more unique if she used her Naginata rather than her great sword. I can imagine her summoning walls of flame or tossing the weapon like she does in the game for some sort of ranged damage. Final Smash could be the Naginata's special move in Hyrule warriors, or something else Sheika related.
Captain Toad could be funny, with his heavy backpack he could be a very small and yet at the same time heavyweight character. Not flying as far when knocked and having pretty terrible recovery as a drawback. He could use the pick-axe and his back-pack to attack among other things from his game. For a final Smash he could lure that big bird guy (name forgotten) to attack an enemy by planting a power star on them. (That or something with Toadette, though maybe toadette could be an alternate skin instead?)
Finally, Zael would be really cool if he could incorporate the way he attacks in his game into Smash Bros. Basically, he could summon magic circles depending on what special he uses and when he does his Smash attacks within these circles they gain specific elemental attributes and effects. A similar mechanic to the way the magic circles work in The Last Story. It could be very unique and fit in nicely with the unique aspects of his "cousin character" Shulks Monado mechanics. I'm not sure about what his Final Smash would be... maybe something to do with drawing in the attention of attackers? I'm not sure.
Regardless, this was my rant about my hopes and dreams for Smash DLC, and this was only a small portion of them too.
tl;dr
I like the idea of Sceptile and Bayonetta but neither seem likely. I'd really like to see Lana, Impa, Captain Toad or Zael get added!