Miis are basically Smash fighters already. Not only do they sport the Smash logo, they also have no moves originating from the Wii Sports or Wii Play games.
The only thing the Miis are missing in that regard are Primid/Alloy/Polygon/Wireframe costumes. I know I've repeated it far too many times, but not including them has been a bizarre oversight, considering how well they'd lend themselves to the Mii archetypes, the ease of asset reuse and the sheer volume of Mii DLC costumes that have been produced so far. Would more people really buy that hideous flame T-shirt costume over fairly prominent original enemies from prior games?