Technically, R.O.B's not a
character, though; it has no personality, motives or really anything to make it a character. I'd also like to point out that Prime's appeared in the same game as Snake and Simon Belmont, actually; it's a Japan-only game for the Gamecube and PS2, and was a joint venture between Takara, Hudson and Konami...it was multiplatform, but
still.
And yes, IdolMasters is a game; but it's also Japan-exclusive at the moment and highly obscure. And if we
REALLY want to be technical, Takara's also made multiple games of the series only on Nintendo consoles; two for the Famicom/NES and there was
going to be one made for the SNES by Argonaut Software using the Super FX chip (but that later evolved into the game
Vortex), which would have probably looked similar to the original
Star Fox.
Sakurai knows Smash has a big Western following, and if we try to, he may just think about doing it. Even if Smash is to celebrate Nintendo's history, you kinda lose that once you start seeing characters like Goku and Shrek skyrocket up polls. At least Prime is somewhat possible, if not very likely at all. I mean, I fully admit that the likelihood of me seeing it happen is pretty much
zero. But I'd still love to put some support in regardless, and at least there's some chance of it, unlike with other characters in the same boat.
Again, we
know Sakurai takes input from Western fans; we'd never have gotten Little Mac if he didn't. So if he sees people want a character, he'll look into the idea of using that character, regardless of what we try to reason. If he sees Optimus Prime on the polls, he'll consider the option, even if not for this game, or even the next. He'll still have the idea, regardless. Smash is as much a marketing ploy as it is a celebration, so considering how lax Japanese companies are with licensing, I could definitely see it happening if Sakurai decides to try it.
And without the wild speculation, where's the fun in the Smash speculation scene, anyways? These left-field ideas are straight-up Sakurai's alley, even if not for the Ballot.