even though its hard tpredict we DO have the patternspresented in smash 4 to base theories on.
every single smash 4 newcomer had a new gimmick, bar, or mechnic entirely different form anyone else.
wii fit could self heal, mac ko meter, bayonetta and ryu input moves, cloud limit, robin (resource usage),corrin first character with a true stance with options, villager pocket aka permanent item maniplation, dhd and megaman projectile smashes, palutena custom specials (RIP).
i do expect to ee charatcers added that continue this trend which points me more toward characters like dark samus (phazon), shantae, geno, or paper mario ( i can see him getting some unique mechanic)
additionally splatoon characters will more than likely have a fuel mechanic or a movement in ink mechanic and that supports this as well.