Your ideas for items have already been done.
There is a pokeball Pokemon in Brawl that functions like your metapod idea, except it is Bonsly.
For the magikarp thing. The cuccos from Zelda, and the beehive from Animal Crossing essentially do that.
Though I really like machamp a lot, I am also planning to main Machamp and Lucario in Pokken btw, my main personal picks would be Tyranitar and Sableye. Ttar would be a heavyweight that would draw a lot of inspiration from Godzilla, and Sableye would be a troll who would have a gameplan designed around status ailments. Toxic to do damage over time. Will o Wisp to diminish the opponent's damage output. Locking moves of of the opponents moveset with taunt, like maybe use it to disable shielding.
But if Sakurai came knocking at my door and asked me who I thought the best smash candidates from the Pokemon universe would be, I would say Eevee, Blaziken, Gardevoir, and Blastoise. Eevee could be a stance change character with multiple forms and attacks based on her different Eeveelutions. Her dash attack, recovery, and dodge rolls could involve her turning into Jolteon and zip around. Shielding woud involve her using Vaporeon's acid armor. There are just a lot of ideas you could go with for the character.
Blaziken would be a tae kwon do character with lots of rushdown and rapid hitting kick moves. Gardevoir would be a campy character and function like Zelda or Palutena. And lastly Blastoise. Blastoise was actually considered for brawl to compliment charizard, but Sakurai decided to go with Ivysaur and Squirtle over him. He could use his canons to jet himself around as well as pull off rocket punch and tackle attacks.