Since I've played with Joker a bit, and was browsing the forum and saw this topic. I realized that Joker plays very VERY similarly how I imagined Ghirahim would play in Smash, minus the persona (though Ghirahim does have multiple forms). Tall thin speed character with a knife who can counter, and casually tossing a tiny projectile at you at a slight angle. Just replace a few of the kicks with his sword attacks (then again Link kicks in Smash and he doesn't do that in the games), and replace grapple with his teleport. I could easily see Smash 6 replacing Joker with Ghirahim if they wanted to have someone joker-esque without it being Joker. However since Joker is here, Ghirahim feels a bit redundant if he got in.
Also now looking around that people mentioning characters that weren't in the list. Ray MK III would be pretty neat if they got in smash. Smash fast character with projectiles sounds like a fun time. Skull Kid would be fun, as long as it's not like Hyrule Warriors where he plays a note on the Ocarina for every single gosh darn attack! (So salty about that. He was so fun to play, but I can't stand the ocarina noises!).
However, the big one for me would be Shadow. In fact, he's probaby one of the few characters that exist that could be a third loyalty main for me. Looks cool? Check! Like the personality? Check! Do I have memories or history with the character? Check! So he definitely has motivation for me to main him. It would come down to what his gameplay is like. However, Sonic in Ultimate feels just so perfectly natural to use, so if Shadow was an alt costume or echo I know I'd be right at home playing Shadow. However if they gave him more unique features like teleporting to recover, having chaos spear as a projectile it'd be nice. He does have unique punches+kicks and such in the sonic games if they really wanted to draw from the sonic games even more. Even if his gameplay was pretty bad, I probably would be stubborn enough to get over it because of how much I like Shadow, just like I did with Meta Knight (and to a lesser extent Lucario). I'm mostly worried about him becoming a heavy hitting slower version of Sonic, even though he seems to do more combo type attacks in the games compared to sonic from what I've seen. I could easily see Sakurai making him have not much traction because of his shoes though, which might be either annoying and/or interesting with such a fast character. So yeah, playable Shadow in Smash please!