Well I don't think Heihachi, Mega Man X, Zero, the Virtua Fighter dude, etc.. will ever be playable characters so I wouldn't hold my breath for King K. Rool to be playable.
Because they are in the same league as King K Rool? Never said if they are a mii costume, it means they have a good chance to be playable... Im saying weither or not King K Rool is a Mii Costume or not, he has a huge chance to be playable still. Having this costume changes nothing.
King K Rool definitely is the most popular character in the polls (especially from 1st party characters), if he isnt in basically the poll is BS lol