If it's possible (it probably isn't) to add Free for All's to Showdown, I think that it should be done.
I think that FFA's would be successful on Showdown because using Multi Battles as a platform for FFA's has been popular on Twitch for quite a while now, showing us that there's already a playerbase out there. FFA's on Showdown also wouldn't have the problem of one "side" being able to win together, to be unable to target certain players with certain moves, etc. Random Battle FFA is also something that could give Showdown FFA's more unique appeal.
I know that Showdown is primarily geared towards competitive play, but like Random Battle, FFA has a healthy casual appeal and can be a lot of damn fun.
Thanks for reading! =)