Ah yes, the one thing that prevents his son Kirby from getting representation from the modern games.
(Sakurai created Kirby.)
That sounds stupid i know, but it is the truth. Sakurai may have created Kirby, but he hasn't had a major hand in the franchise since Air Ride. (the latter of which inspired Smash Run from 3DS)
Have you noticed how we have never gotten a stage from Kirby 64, or Kirby and the Amazing Mirror, or Dreamland 2 and 3, or Return to Dreamland or Squeak Squad or Triple Deluxe or Planet Robobot or Star Allies? We have only gotten stages from Dream Land 1 (which is the only Dreamland he was directing) Adventure and Super Star. The closest we got was a scrapped Epic Yarn stage that was retooled into the Wolly World stage (which honestly sounds more like an idea scrapped early on, and i think the only reason Sakurai mentioned this is because he wanted to save face)
We have gotten content from Modern Kirby games, namely the soundtrack has had music from Return to Dreamland, Kirby 64, Amazing Mirror and the like (ironically, there's not a SINGLE track from Epic Yarn in the series, which is why i heavily doubt the supposed Epic Yarn stage even made it to planning stages) and Kirby and Dedede's Final Smashes are based on RtDL and Super Star Ultra respectively, so at least there's SOMETHING, which is something i can't say for poor Wario Land, which has been neglected in Smash for ages (right down to having its most iconic element removed in Smash 4 and bought back in Ultimate when it souldn't have been removed in the first place) But even then, it's pretty clear where Sakurai's priorities rely on games that he worked on, i mean Ultimate only further supports this, with the Assist Trophies being Knuckle Joe and Chef Kawasaki, both debuted in games that Sakurai had a hand in developing on as opposed to say, Adeleine, Taranza, or Susie. Even the boss from the Kirby series proves this. He chose Marx, a character that debued in Super Star, as opposed to someone more recent like Magolor, or Queen Sectionia, or someone more prominent like Dark Matter (who is the main antagonist of his own entire trilogy, as opposed to Marx who is the main antagonist of ONE mode of Super Star) Aside from music and Final Smash nods, Sakurai really doesn't use a lot of Post-Air Ride or Shinichi Shimomura (director of DL 2, 3 and 64) content in general, and any examples of it being used are exceptions to the norm rather than being...............well, the norm.
This isn't me just trying to be pessimistic for the sake of it, because it's really common among Japanese game developers to limit themselves to stuff they created, as opposed to using stuff created by others (which in know sounds really stupid seeing how Sakurai works with multiple franchises that he never worked on before Smash such as Sonic, Castlevania, Mega Man and the like, but hey i never said this line of reasoning made much sense, i just say that it exists) Franchises like KH and MK are also examples of this mentality.