As far as villains go, I think there are a few that stand a chance.
1. King Hippo (or any other opponent from Punch-Out!!, though he is definitely up there)
2. Captain Syrup (granted Sakurai has a weird ignorance of Wario Land - either willful or not)
3. Dark Samus (if Sakurai wants to add a third Metroid rep, I could see him going for her)
4. Medusa or Hades (also Viridi sort of)
5. Fawful (though I have bias)
6. King K. Rool
7. Ganon
8. Samurai Goroh
Nah man, the next slot should go to....K. Rool.
For some reason.
I guess I can kind of understand people thinking certain characters are more deserving of a spot in Smash than Shovel Knight (I respectfully disagree) but anyone saying it should K. Rool aren't making their case very well. That pick would be only slight less WTF than Geno or Goku.
Oh come now, there's numerous valid reasons why people think K. Rool deserves to be in.
1. DK rep (four games and only two reps for one of the biggest video game series ever?)
2. Villain
3. People want Trouble King 3
4. People have wanted him for a really long time, and his popularity has been pretty decent at worst TBH
5. Villain of a series considered a classic to many
Really, the only good reason against K. Rool is a personal one of "I don't like him". Moveset potential ain't a thing (and in K. Rool's case not even reasonably a problem), relevancy is only somewhat valuable (in that it may influence Sakurai's decisions), etc.