Ridley would be cool, and is by far the most obvious Metroid character to include (I mean, Mother Brain or Kraid would honestly not work, and the Hunters from Corruption are all equally non-fleshed-out-enough) but I don't think he'll make it in. He is way too big (he is a fucking SPACE DRAGON after all, and if you tried to shrink him down to, say, Bowser-size, he would look too funky) and it would be hard to give him a lot of moves. I mean, he can basically bite, claw, fly, and maybe breath homing fireballs, but that's it.
I just can't picture him in the game. I don't think he'd work out, on multiple levels.