I'll play devil's advocate and say that Quote or Curly or any representative from Cave Story should only go so far as an assist trophy.
My perspective being, what makes Quote unique is his ability to fire various weapons in any direction. In the perspective of Smash Brothers, doing this would indeed make him very unique but potentially boring or unfair to other fighters. Not doing this would fail to emulate the sort of skill that comes requisite of being an avid fan or player of the game.
Megaman, for instance, plays like you're playing Megaman. Kirby plays similarly to a Kirby game. Many characters are invented and designed in such a way that they feel like they are indeed that character. But if I were playing Quote in Smash Bros... Why would I ever use silly weapons like the Spur or Machine Gun, when it's much more efficient to rush up close and spam the Blade, like I would while actually playing the game?
Which leads to another sad counter-argument: the fun behind Cave Story is the ability to change your multi-directional weapon on the fly and use it in the unique and goofy way you see fit for the situation. Megaman gets away with his similar case due to simply having weapons useful to those particular states of input and animation. I can't honestly say the same about Quote or Curly Brace.
I'm sorry, but my first vote was simply Shovel Knight on this, and the simple difference is Shovel Knight having existing melee abilities, and his special weapons actually acting differently. Quote? Aside from the Fireball, which is truly a momentum-based weapon, almost every weapon consists of shooting in a straight line. If there were more weapons like the Bubbler or Snake, that might be a different story, but... 4 or 5 actually unique weapon types feels really rather empty to me. I simply can't picture Quote represented properly in Smash Brothers.
So, assist trophy idea: Have Misery appear on the battle field and act out her boss routine for one cycle. Drops a falling block on someone's head. That'd be really cool.
