If I was to make a roster, I truly think about who would be worthy of being playable.
There are three things I want form a playable character in a platform fighter. Mobility, Arsenal, and Combat.
Mobility: I'm not just talking about being quick and nimble. The character has to have something that can propel itself anywhere, especially upward. Either a skill(the SMB2 Super Jump, Cammy's Cannon Spike/Thrust Kick), a Power-Up(the S3&K Electric Shield, the Hi-Jump Ability) or an item(Proto Man's Proto Coil, a Jetpack) can work for many. This is primary why I'm having trouble thinking up something for Simon Belmont, and would rather choose Richter Belmont instead.
Arsenal: Of course they have to have something to dish out some major damage(or at least, some trouble like confusion or paralysis). Either weapons, elemental powers, or whatever they can pull out that would make good, workable special moves. This is primarily why most characters from Fighting Games and Beat 'em Ups are hard to work movesets for(as well as the fighters being considered boring. They fight and nothing else), Especially 3D Fighters(Praise the Lord for oddballs like Yoshimitsu).
Combat: Though you can unleash the biggest fire power you want, it wouldn't matter if the enemy is that close up to you ready to pound your face in.I still say you need to have some good form of close range combat for minor moves(weak and strong). It'd be good for setting up combos. Either punches and kicks(Chibi-Robo,Twinbee), scratches, slashing, and clawing(Corrin should be able to do this), or any close range weapon(Farfetch'd's leek, Waddle Doo's sword for "Kirby: Right back At Ya!, even Mappy having a night stick) will work for this situation. This is Primary why thinking up a good moveset for say, anyone from Contra, is extremely difficult.
To give a good example, The titular Legendary(yeah right) Starfy(as well as his baby sister, Starly) has Mobility and Combat down, but he(and she) is(are) lacking in the Arsenal department(I don't want to give them any abilities that alter their appearances)
These three are what matters the most to me. If a character fits all three conditions(as well as having a major appearance on a Nintendo System), then they certainly qualify for my roster.