Bolded are ones I disagree with for reasons.
Tom Nook - Discussion has been had on this guy before. He'd be interesting. Maybe a bit difficult to make, but he'd work out well.
Ridley - Aside from being too big, he's a must have. Or perhaps just use Meta-Ridley so he can have missiles and whatnot.
Toad - Toad is already in the game. Peach uses him as a human shield. Because that's all he's worth. I've never been too excited about Toads in anything. Might as well use Kremlin or Brigand (from FE) as a character.
Bowser Jr. - Paintbrush ho! And more SMS love is always good.
Tingle - Everyone is fine with this choice unconditionally.
Ghirahim - Iffy here. I think we'd have to see more of this guy. After all, he's another 'sword user'.
Zoroark - Ew. Pick a good Pokemon.
Isaac - Good choice.
King K. Rool - No issues here.
Samurai Goroh - Convince me otherwise with a possible moveset.
Lyn - I feel like she'd turn out being a mix of Marth and Ike. Marth for the litheness, Ike for his side-B. I love Lyn to death, but the sad fact remains that other Fire Emblem characters would fit in better. Micaiah and Sothe (God forbid they make it in), Hector, hell, even Nephenee would be better on a gameplay standpoint.
Little Mac - I know he deserves it, but all I'm seeing is 'Captain Falcon clone'.
Diskun - He'd be fun.
Mega Man - Definitely.
Cloud - Too close to Ike/Dedede.
I'm also a big supporter of Lanky Kong.
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Don't worry, Bill Murray!
Its things like this that make you wonder why the company that makes M&Ms don't make butter