As discussed in the Main Roster thread before it got shut down, we realized that Nook is not "the" icon/mascot/whatever of the series.
There really IS none.
Nook is just highly recognized, but so is Resetti, K.K. Slider, Rover, Rosie, Kapp'n, Tortimer, etc.
I agree with you that there is no
one icon, but rather a group of highly recognizable characters. I would say that Nook, K.K. Slider, and Resetti are the most recognizable to those who do not play Animal Crossing. Nook has somehow amassed a reputation for being a loan shark, mafiosi, and/or a money launderer (and this is the impression plenty of people have of him), K.K. Slider is known to for Totaka's song, and Resetti is a humorous character whose spiel is pretty well-known. Other than those three, I doubt the average non-AC player would know who Rosie, Tortimer, Kapp'n, etc. are. That doesn't mean it's impossible for them to know them; my mom knows Jigglypuff, Poliwhirl, and Bellosom, but the Pokemon most people are most likely to be acquainted with is Pikachu (maybe Meowth as well due to his prominence in the anime).
As such, I would say that those three plus the villager are really the only viable choices for an Animal Crossing Character. Resetti has the AT slot occupied while K.K. Slider has his Saturday night show. I wouldn't count them "out," but I would say they're less likely than Nook and the Villager. Between those two, it's really a toss-up depending on what Sakurai and his team think. I honestly don't think there's really any evidence pointing in any direction right now. The only hope we have is that Sakurai appeared to renege on his statement that AC characters can't fight and/or the series is too peaceful. There are also supposed comments from the co-creater of the AC series (though much like Nook's supposed fight club, I have yet to find a reliable source for this) which could hold back any AC character even if Sakurai were to have changed his mind.
Ultimately, this is a guessing game. The situation has changed a bit since Brawl; Animal Crossing is a
huge franchise that even people who aren't gamers enjoy playing. We know Sakurai wants to encourage beginners and advanced players to play together. What better way than a character from a familiar series? Of course, you could argue Mario takes that role, but it's one thing to play as the mascot of a series you don't really interact with often and another thing to play as a character you've "talked" to, interacted with, grown plants with, etc. Animal Crossing's laid back and non-combative nature, which proved to be its death knell during Brawl, can actually be reinterpreted to provide a sort of friendly beginner character to those who've never played the Smash series before. Put a bit of the cartooniness back in Smash, let's say.
Once again, pure speculation! I have no evidence to back these ideas up.