Animal Crossing is doubtlessly one of the more interesting series to think about when it comes to the next Smash.
Historically, AC was always eligible for a character as early as Brawl. However, Sakurai has admitted that he did not find the series capable of producing a fighter at the time. Only when the For games came out did Sakurai get inspired enough to make a move set by screeching things a little for Villager.
Isabelle would be made for Ultimate, but she would end up being a semi-clone. Whether due to lack of remaining new ideas for the series, a development time that left little room for unique newcomers, or some combination of both, Isabelle ended up with a move set that feels more like the leftover scraps of Villager's ideas rather than a move set that is actually feels like hers.
The clone stigma of Isabelle has unfortunately continued on to the remainder of the Animal Crossing characters. While it certainly could afford a third character, it's harder to champion a character from Animal Crossing with enthusiasm when the series is already screeching it's own move set possibilities thin and likely to produce another clone. The fact that its a non-combative series with a gameplay loop fairly well represented by one character and who's only other character is a clone whos uniqueness is somewhere between Ken and Toon Link does not inspire confidence that a fully fledged, unique character is coming soon.
It's situation honestly resembles that of Star Fox to me, when it ends up with three very similar characters that ended up being clones of each other for a combination of reasons, but ultimately come down to how hard is is to translate their gameplay into Smash. And while Wolf was supported back before he was included, Krystal was just as if not more supported back during that time (pardon me if I am wrong, I wasn't there during that time). And while Krystal still has some support due to how unique she could be, overall support for other Star Fox characters like Slippy Toad and Peppy Hare isn't anywhere near the ballpark of most characters generally accepted as popular.
I honestly think that if Animal Crossing were to get a new character, it'd be a Wolf-tier clone at best for probably Tom Nook, before the support becomes too fragmented to earn a new character.