I do think Isabelle is probably the most likely 2nd Animal Crossing rep if there is one (And I don't even think that's necessarily a guarantee yet). Though I usually see Isabelle fans kind of conflicted about her being playable. Some may not want to hurt the little secretary girl like that. There are cute characters in Smash though the difference is their main gameplay was about fighting eachother (Pokemon, particulary Pikachu and Jigglypuff), characters that look cute but are kind of monsters that can go to the lengths of EATING hordes of enemies (Yoshi and Kirby), and/or went on an entire adventure battling enemies along the way (Toon Link, Ness, and Lucas).
None of them are simply just pacifist assistants who just try to do the best they can to help out the player's avatar (Well except maybe Wii Fit Trainer in trying to get you to get fit, but I think that's another matter entirely)
As her current assist trophy in Smash 4, you can't hurt her. Worst you can do is make her sad if you KO the character she's trying to help out while she's still out there. Some may feel that's more in line with her assistance character.
I'm neither for or against Isabelle. Just noting that there may be some amount of hesitance about it even among her own fanbase heh. I suppose on the flip side it'd work towards some degree of humor to see her manage to beat up all the other fighters whichever way she fights. Still I couldn't blame fans for not wanting to see her get hurt in the many ways she could in Smash heh.
I'd still welcome it if it happens just because she has become the face for newer Animal Crossing titles. But I wouldn't be surprised to see alot of Isabelle fans after the confirmation say "Oh this is so cool! ...But I don't want to hurt her
".
Villager was a blank slate and was probably helped by the Killager meme to kind of make him fit early in Smash 4, and AC Fans have always had at least sort of a beef with Tom Nook for all the debts he makes you pay in the series so there probably would be less of a hesitance there. (I'd laugh if Tom Nook was a DLC character. Making you pay REAL money to put him in the game, would be kinda meta that way). Not saying I'm explicitly supporting Tom over Isabelle though. I'm fine with whatever they do with Animal Crossing representation. I only played New Leaf so I'm kind of just a casual fan of it (Haven't touched it much lately though, briefly picked it back up when the Welcome Amiibo stuff dropped but lost interest again pretty soon afterward)