Zoroark could be a great addition, although it seems kind of redundant what with Weavile and Lucario. Still though! Its Illusion ability could make for some interesting maneuvers.
A serpentlike Pokemon would make for an unexpected addition as well. Since Pokken Tournament seems to have a habit of adding weird and unexpected Pokemon, why not Dragonair over Dragonite? It could hover with its wing ears and shoot energy bursts from its gems. Also, as a snake, it would also be able to wrap itself around the opponent. It would be very fast and agile.
Also, more Electric-types would be neat! Why not Ampharos too? It has a Mega and I could totally see it fitting into the roster. A fully evolved Electric-type would be a nice addition.
For a bulkier alternative, Chesnaught would be pretty great too. Not just as a Fighting-type, but because its Spiky Shield lends itself to interesting and unique strategies I could see being implemented into the game.
Oh and we need a Fairy-type too! More quadrupeds as well... I think Sylveon would be a great choice too. As representative Eeveelution in the game. Like Suicune, it has strange ribbonlike protrusions that could lend itself to fighting in this sort of game. I could see it as being a very stunning Pokemon. As in, it uses its ribbons to grab and stun you with its affection for a bit.
I know some of these are probably assists, but they would still be interesting choices to consider.