With the advent of Shadow Mewtwo, Pokkén Tournament's roster is pretty much perfect in my eyes. I look forward to demolishing my opponents with shadow-infused psychic powers! Shadow Mewtwo is definitely going to be my main. However, my second-favorite Pokémon of all time, Dragalge, would also be a welcome addition. I've posted my idea for his playstyle here before, but I'll just do a quick recap for the sake of those that haven't seen it. Basically, Dragalge's long-range attacks would be Poison-type moves that inflict continuos damage on opponents, allowing him to pressure opponents into approaching; while Dragalge's close-range attacks would be Dragon-type moves that pack a serious wallop, allowing him to doll out high amounts of damage after pressuring the opponent into approaching him. However, in order to counteract his powerful pressuring game, Dragalge would probably have to either be really slow or have exceptionally poor HP, or perhaps a milder combination of the two. That's my spin on him, anyhow.
Other neat fighters could be Aegislash, who could utilize its unique Stance Change ability in battle, Magneton, whose three-in-one nature could make him an extremely distinctive fighter, Hitmonlee, whose elastic arms could give his attacks a wide girth and really make him stand out, or Cofagrigus, who would be interesting purely because of his multi-ghost-armed physique and levitating abilities. I suppose Ditto could make for an interesting fighter, as well, provided his transformation abilities are pulled off right, and I'd also like to see Bidoof, if only for the tidal waves of salt that would inevitably follow
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EDIT: Also, Mew. Her vast spectrum of available moves gives her a lot of potential in a game like Pokkén Tournament. I can see her using various types of attacks in battle. Perhaps she could be the "Jack of All Trades"? You know, always bronze, but never gold? I'd like to see something like that, personally.