Samus could definitely use some big tweaks to be more viable and generally interesting to play as, especially since, over the years, Mario, Link, and Bowser, among others, have gotten some. Although, it's probably not going to look like as big of a priority to people as changing up Ganondorf or Sonic.
As for "fourth-party" characters, I have my opinion that Smash itself shouldn't get them, but if Nintendo wanted to do a side game of Nintendo VS Disney, Shonen Jump, Nickelodeon(!), or WB, etc., that could be interesting. I feel like Sakurai has some familiarity/affinity w/some of those series, like JJBA, the MCU, Star Wars, etc. as far as I remember. But then again, I'm unsure of him working on this kind of stuff for a while, and so is he.