I think I may be the first person who realized this! pic.twitter.com/mQGQPNAzjc— Ethan Mouta | CrashForSmash (@Ethan_Mouta) June 13, 2019
Okay, this is a pretty fantastic find, and it certainly seems like something Sakurai would do.
First up, we have the classic trophy description for King K. Rool, which ponders whether Donkey Kong and King K. Rool could have been friends at some point. What on earth could make King K. Rool actually change his heart and become friends with DK?
In one of the Smash Bros. Ultimate trailers, you see Joker from Persona 5 stealing the crown of King K. Rool. For those who don't know, Joker is part of a "group known as the Phantom Thieves of Hearts whose purpose is to explore the Metaverse, a metaphysical realm consisting of the physical manifestation of humanity's subconscious desires, in order to remove malevolent intent from people's hearts."
The most recent trailer to feature King K. Rool was the reveal of Banjo-Kazooie in Smash Bros. Ultimate. That trailer starts off with Diddy, Donkey, and King K. Rool all hanging out at DK's house. I did think it was pretty odd at the time that King would be hanging with Diddy and DK, but I chalked it up to Sakurai wanting all the RARE characters to hang out together.
Now with the info above, it seems there might have been more to it! Did Joker remove the malevolent intent from King K. Rool's heart?!