Metroid as a series is one that thrives off the isolationist mood of it.
Because of that
is one of few characters that really has any impact for the series.
being her in a different form which functions and looks entirely differently, in my opinion fits.
The issue is, there are only 3 other Metroid characters with damn well near the impact to even be considered.
1. Ridley - The main villain against Samus even if he isn't the head honcho of any operation. Tied to her PTSD and always that last boss you fight for your ultimate power for the main game.
2. Dark Samus - Embodiment of the massive success of the Prime Trilogy, a dark clone of
with an entirely different arsenal. More akin to
than
.
3. Mother Brain - THE Final Boss for Metroid as a whole, despite being it for only 3/4 games. However, trying to craft a way of making her fit as a fighter is one I don't think Sakurai would stretch for.
Sylux is the character Samus has interacted with for a "humane" way but he has far too little impact on the series to be valid, imo.
And look at that, all 3 of our big forerunners have their role in Smash 4 which is heavily suggested to keep them out for this time around.