Hmmm, might as well contribute.
1+
Lyn
Im a sucker for Battoujutsu/Iaijutsu swordplay, and what we've already seen doesn't disappoint. Its like PM put Vergil in Super Smash Bros. No doubt they took inspiration from
this.
Isaac
You can't mention GBA games without mentioning Golden Sun. With Gaia Blade Unleash attacks (Power Charge, Spiral Assault, Sky Blade, Titan Blade), unique Psynergy (Move, Retreat, various Earth and Plant based attacks), and Venus Djinn, its crazy Isaac was never included as a playable character. Since Felix shares many attributes to Isaac, he could double as a alternate costume similar to Robin in Smash 4.
Ridley
Not a big fan, but I'd be lying if the moveset potential wasn't there. I imagine his Analog Movement is similar to
Aliens and attacks and special moves are reminiscent of Firebrand. As for him being to big... If Capcom can make Sentinel work in Marvel, I'm sure PM could find a way.
1/2
Pokemon Trainer
Doesn't need to be his own character. Have him interact with Charizard, Ivysaur, and Squirtle like he does in Brawl.
-1
Young Link
Brawlvault is your friend.
Pichu
The Baby Mario of Pokemon. Terrible character design, unoriginal moveset, and design mechanic where he deals damage to himself. Even the devs at Nintendo mentioned Pichu was a joke.
"But the PM team can give him the Roy treatment!"
And? He could be the best character since ST Akuma, nobody wants his inclusion besides the 5 outspoken users in this thread who keep circle jerking each other. Its funny, cause the same people voting for Pichu are the same who whine about Dr. Mario, Lucina, and Dark Pit.