As for playable IPs new to Smash, I personally see (in order of likelihood IMO) Punch-Out, Xenoblade, Golden Sun, Starfy, and Sin & Punishment as possibilities. Advance Wars definitely merits a character, but I doubt they get one. If they had gotten a game between Brawl and now, I'd have also included Custom Robo. Any other IP is probably best off as an AT or trophy. (Not counting AC ofc)
For the retros, I think Takamaru, Balloon Fighter, and Mach Rider are the frontrunners (with Takamaru ahead by quite a lot), though many other retros could definitely work. However Takamaru and Mach Rider would probably be best for a "revival" in the same vein as Pit. Balloon Fighter just seems to have a decent amount of popularity (for a retro) and potential, as well as the fact that Nintendo seems keen to emphasize that IP above other retro ones recently.
As for the original topic though, I'd say Nintendo still has enough notable unrepresented series to use before relying on third-party overindulgence, which I doubt Sakurai would ever condone anyway. Smash Bros is primarily Nintendo, and even with the addition of third-parties, Nintendo will always come first in Smash. I'm not saying this as a loyalist or a fangirl, I'm just saying it because it's the truth, even without Sakurai (who - even if he says differently - will most likely always choose to develop Smash over someone else doing it) Nintendo won't condone loading up their game with characters they have no control of. Nintendo x some other company might happen one day, but Smash isn't it.
Also, obtaining the rights to Banjo would hardly be as easy as rights to other third-parties (as Banjo isn't a third-party character).
Oh, hell no. Sheriff never used the Zapper. The Zapper didn't exist in 1979. You didn't need a light gun to play Sheriff. Duck Hunt Dog gets dibs on that Zapper!
The NES light gun might not have existed until the eighties, but Nintendo did have a light gun out since at least the early seventies that worked with, believe it or not, a sheriff-looking character (the Custom Gunman). Considering, if he was added, Sheriff would be a throwback to Nintendo's early days (and would be the oldest character included), I could definitely see Sakurai making a reference to the Custom Gunman (actually one of Nintendo's more well-known toys) in Sheriff's moveset. Either way, while Sheriff himself never used a light gun, it wouldn't be totally without precedent and context if he had a Zapper.
[COLLAPSE="Custom Gunman"]
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Sheriff isn't a very good idea. While it's true that adding a cowboy along with a western stage and music would add something new to the series, there's nothing really... interesting about it. No twists on the idea. He's just a cowboy doing cowboy things. And he doesn't do anything other than shoot in his games, so everything for him would be made up. You could literally pick any standard cowboy character ever and get the same experience that Sheriff offers.
He's nothing special. Nothing worth adding.
I actually agree with this. While the character definitely merits inclusion as the token "historical rep", I honestly have no interest in Sheriff, to me he doesn't seem very exciting. Just my opinion though, I'm sure others disagree, and hypothetically Sakurai could make him a great character like the others, I just don't think he'd be as interesting as G&W or ROB.