My little take: He may not have appeared in any games recently, but King K. Rool is still more of an all-star and thus, suitable for a game like Smash Bros Ultimate. He is more suitable and much of an all star than any other villain in Donkey Kong games, from the Kongs from Jungle Beat to the villains in Returns and Tropical Freeze and so on. He is still relavant as a villain, just not perhaps in the recency kind of way. Plus, to me, he has more all-star power than the likes of Captain Syrup from Wario Land, Shovel Knight, maybe Shantae, perhaps Issac from Golden Su, Bandana Dee from Kirby, maybe, and others like newer guys like Spring Man from ARMS and Rex & Pyra from Xeboblade 2. (no offense to fans of those guys, of course). So I don't think his chances are too hurt in terms of getting in. Plus, if Ridley can get in because of fan demand, why not King K. Rool? I know Ridley was more recent than K. Rool, but they're both heavily wanted characters to begin with, and Ridley's relevancy and recency at the time of games like Smash Bros Brawl or Smash Bros Wii U and 3DS did not get him in as a playable character, but as a boss. That said, King K. Rool did not get in the last game as a playable character, but he did get in as a Mii costume. So Sakurai does know K. Rool has fans. Plus, for a guy made by westerners, he still is very popular in even Japan. I am not saying K. Rool has an absoluitely sure chance of getting in, but he has still established himself as an all-star, so regardless of recency, he still has good reason to be a playable character in Smash Bros Ultimate.