Pokken has gone down on record saying it has no plans for DLC and additional characters, but of course that doesn't mean they might decide on DLC down the road. Plus, Sakurai said the same thing early on in Smash's life and now we have Bayonetta as our 7th downloadable character. Pokken has 16 Pokemon in its roster, with 2 pokedex entries repeating through Pikachu/Libre and Shadow/Mewtwo. There's also some interesting diversity in design with a quadripedal frost beast, a haunted chandelier, a mouse dressed as a luchador, a dragon, an earth dragon, and an icy weasel. So there stands to be more choices to introduce should the unlikely/likely event of new DLC characters to expand the variety in fighters. The choices listed below are my own and chosen through a combination of Pokemon design, gameplay from their turn-based origin, and possible fighting game design. There are over 750 Pokemon and Pokken's even proved that not-fully evolved Pokemon could even be considered thanks to Braixen's design as a pop star witch.
These two Pokemon fill a my personal choice for more steel-type representation. Though the two of these Pokemon would be designed differently, I wouldn't pick the both of them at once since they both still fill the general spot of steel-type bruiser, most likely a power-type fighter.
Aggron would be the heavier of the two choices. Aggron's best known for his insane tanking and defensive abilities, usually taking a role of bruising through physical attacks and returning that damage reliably. In my design docs, Aggron would be exceptionally slow but would have many moves with armor built in. Notable moves include Iron Head (good contender for armored charge move), Iron Tail (probably a good poke), and Metal Sound (probably good excuse to give Aggron a projectile in field phase). Could you imagine a burst super that is perhaps a superpowered, high altitude heavy slam? Or flipping a huge slab or stone onto his opponent before performing Head Smash?
Metagross on the other hand, would excel at more at more unchecked aggression. Metagross is known for strong defense and sweeping with strong physical attacks after an agility. Metagross wouldn't have armor like Aggron because Metagross would be much faster despite being a power type, not only because he would obviously have Bullet Punch to charge forward, but because Metagross is typically depicted levitating its whole body using telekinesis. In addition to Bullet Punch as a high speed charge, he would have Hammer Arm (probably high power, reduced speed), and Meteor Mash which has never been clearly represented. It could probably be a midiar special attack. where he'd come crashing down as a heavy attack.
This bug/steel Pokemon deserves a spot free of the consideration for a steel-type Pokemon for Pokken. Scizor has always been a great revenge killer, using Bullet Punch, U-Turn, Pursuit, and Bug Bite with it's great strength in order to scare off opponents afraid of Scizor. Other moves to consider for Scizor's specific design include X-Scissor, Fury Cutter, and Fury Cutter. It would be interesting to give Scizor access to Swords Dance considering it's a standard.
Scizor should be a standard type with a wide variety of fast moves to which lend itself to fast paced, high pressure rushdown much like Weavile. Despite not really being particularly fast in Pokemon, it's infamy for using Bullet Punch and U-Turn makes me feel that Scizor should work as a speed-type fighter.
Personally I find it interesting how despite being the same Pokemon, Pikachu and Libre play differently and Mewtwo and Shadow Mewtwo are extremely different. Gallade however isn't very similar to Gardevoir, being an entirely different Pokedex number. Still, adding representation to the interesting Ralts family. Gallade's always been an interesting design too, being a chivalrous knight who lives with a blade for an arm. I'd like to imagine Gallade's long-range options to be limited but to have many ways to gets in with enough skill. In particular, Machamp's Karate Chop can be used to obliterated projectiles, and Gallade should be able to do so too, considering he's probably use his arm blade to deflect projectiles in order to make the duel a fair fight. Gallade would most likely be a standard type fighter.
Notable moves include Night Slash, Psycho Cut, Close Combat, and Leaf Blade. Unfortunately Sceptile already has Leaf Blade and many of these moves are already used as well but you can bet it would be used differently for Gallade's context of swordsmanship.
This bug Pokemon's had interesting history, being a niche attacker at first then becoming a monstrous all-out attacker thanks to its Mega Evolution. But one look at it's massive horn/claws and you know that Pinsir is made for gripping and grappling. Notable moves include Vice Grip, X-Scissor, Storm Throw, and Guillotine.
Pinsir is well known for only being viable for its mega form, so Pinsir could function similarly to Gengar with vastly enhanced stats and moves in mega form with an extra long charging synergy burst gauge. Personally, I think with an emphasis on grappling moves and strong claw attacks, along with a lack of real range choices, Pinsir would make an ideal power-type, particularly if we focus on Pinsir using his head jaws for grappling and throws.
Porygon I think would be an interesting Pokemon to bring to Pokken. Bring him in as Porygon, and build him around a system similar to Junpei from Persona Arena or Frank West in Marvel Vs. Capcom 3. Allow certain moves on Porygon to cause Porygon to invisibly level up until he changes forms to Porygon2 and PorygonZ, since after all these evolutions are a consequence of Porygon smoothing out its graphics and acquiring extra power at the expense of a virus. Porygon could be a technical-type character, with area of denial based zoning appearing at determined areas rather than fireballs that travel across the screen. Porygon2 would find an increase in speed and PorygonZ would have an increase of attack on top of that, while resonance burst would cause all sorts of code and binary to flow out of him while providing the minimum buffs.
Notable moves include Tri-attack, Hyper Beam, Zap Cannon, and Signal Beam. Porygon is also known for its ability Download, which increases the appropriate attack stat depending on the weaker defense stat.
Snorlax has been around a long time and even appeared in the obscure Pokemon Type Wild fan fighting game. Snorlax has always been a fan favorite due to his massive girth and tankiness. And you know Snorlax should have access to Rest, maybe even Sleep Talk. But of course Snorlax will need Body Slam, which should inflict a speed down affliction in Pokken, along with either Hyper Beam or Giga Impact. Snorlax is an obvious candidate for power-type, being a slow, thoughtful, fat attacker.
I think it would be interesting to get a bird possibly in on Pokken to add to the quadripeds and non-bipedal designs. And of all the bird Pokemon, Staraptor is probably the best example of getting a bird to scrap it up in a fighting game context. Afterall, Staraptor is well known as being a bird who can use one of the most powerful fighting-type attacks, Close Combat. With abilities like Reckless and Intimidate with attacks like Close Comabt, Brave Bird, and Quick Attack,I think Staraptor is well suited to compete in a fighting game with a water horse and a chandelier.
All that remains is what Staraptor at neutral would be like, standing still or flapping airborne. Staraptor would more or less be a standard-type fighter, having access to both speed and power. I think Staraptor would be cool functioning similarly to Blaziken, being a high damage, high risk character. While Blaziken loses HP for using enhanced fire attacks, Staraptor would be a more direct recoil based character, losing HP if his highly damage attacks land. Most likely, only attacks like Brave Bird or Double-Edge would inflict recoil but with high damage (think Machamp Close Combat), but Staraptor will have access to other combos and attacks that won't inflict recoil, like Wing Attack or even Close Combat.
Ditto should have just been the placeholder for the random choice really. But what if we decided on an actual Ditto? What if Ditto was an amalgamation style character? Like Edgemaster from Soul Calibur? Or in my mind, Double from Skullgirls. Edgemaster or Mokujin copies a character at random. Double is an amorphous blob of evil that shapeshifts into different characters, using unused attacks from them for an entirely unique moveset. And of course a synergy burst move would be an impressive autocombo of many different attacks from many different Pokemon. With the possibility of having such a complicated number of shapes and moves, Ditto could possibly be a technical-type.
A lesser used Pokemon from the Johto dex is Granbull, a Pokemon that was somewhat recently retconned from normal-type to fairy-type. Granbull isn't used often in comp play due to imbalanced stats but Granbull is known for having exceptional attack. Combine with being a physical attacker with access to Play Rough and you have a unique niche character. Granbull is a bipedal, doglike creature that certainly looks like he'd thrash around with its fists. He's also fairly small, filling the same design spot as Pikachu in being relatively tiny but still packing a balanced punch.
Notable attacks include Play Rough, Close Combat, Crunch, and Outrage (for some reason). Granbull would be another standard type fighter though I admit with his focus on attack and nothing else in Pokemon, it'd be funny if Granbull were a power type fighter.
There's a surprising variety in ghost-types for a fighter with limited slots, with Gengar and Chandelure. Duskinoir is known for its antenna tracking radio waves and using its large hands to strangle victims. I think Duskinoir would make a fun alternative to Gengar except as a slower grappler with the same sort of tricky intangible fading as Gengar. Duskinoir would not only be a strong grappler opposite to Gengar, Duskinoir is also plenty tankier than Gengar with a few tricks up its sleeve, for example Pain Split would be an interesting grab that would perform life steal at a much faster rate than Gengars, as well as several elemental punches like fire and thunder. Mean Look, Night Shade,and Gravity would all also make interesting moves to translate for Duskinoir. Duskinoir would snugly fight in the technical archetype, using the same sort of fading tricks as Gengar to get around and using the same ideas as Machamp to get around predictingthe attack triangle.