User blog comment:Cloverfield monster/Ideas for Alternate Character Skins in the Next Smash Bros./@comment-25275196-20170604013251/@comment-25275196-20170606075835

Dark bowser is the final boss of Mario and Luigi: Bowser's Inside Story. Technically speaking, it's not a form of Bowser, but rather the Dark Star having stolen some of Bowser's DNA, but hey, it's close enough.

If Dark Bowser did get added as a skin, I could only imagine the Mario, Luigi & Bowser VS Dark Bowser battles that would be done.