Unlikely. Smash characters ostensibly have to debut from a video game series or at least be video game adjacent like R.O.B.
It would be weird to change that up, otherwise characters like Goku, Naruto, Luffy, etc could show up and that would also just be odd.
That wouldn't be the first time Smash did something unlikely, though.
Until Melee, Sheikh was unlikely, as she was never playable before (and if we ignore the CDi, neither was Zelda). Until Brawl, Snake and Sonic would've been unlikely too, since they're not from Nintendo. And until Smash 4, it was Cloud, who hadn't even appeared on any Nintendo console in the first place. Finally, until Ultimate, Steve would've been unlikely for being from a non-Japanese game. And as you said, R.O.B. isn't even a video game character in the first place.
So while Dio, Goku, Naruto, and Luffy appearing in Smash may be unlikely, I wouldn't say it's impossible. It all depends on whether Nintendo is interested in adding them, and whether Shueisha would allow it. And we know these characters have been featured in video games before, so...