Alph plays EXACTLY the same as olimar. Literally if he used rock pikmin, stronger attacks,then he would get a character slot(as funny/stupid as that sounds
Lucina-Balanced blade(I didn't see the stream when they unlocked her, so I'm not sure if there is anything else like being faster)
Dr. Mario-Stronger, pills, tornado instead of FLUDD
Dark Pit- PURPLE ARROWS
, diagonal up special(someone correct me on this, hamayama used up b with regular pit and it was vertical, I don't know if you can change the direction), certain moves are slightly different
TL;DR they had slight alterations to their stats/movesets that it warranted a character slot, Alph doesn't have any stat/moveset changes so he is literally just an alt.