You know those like, big empty lots in malls that change their shop depending on season? (Like, become Spirit during Halloween, etc)? That might be a cool feature to bring in all the unique NPCs more, but keep them.. 'rare'. Like, during the winter, Nat might show up once in a while or so with some out-of-season bugs; Chip shows up during the summer doing the same with fish. Katie or Gulliver could have a pop up of unique goods from far off places now and then, holiday-exclusives could have some sort of special collectibles line? Have Jack show up in the middle of summer with some pumpkin goodies, or Jingle around in July xD (Maybe Zipper could show back up around October and carry some basic candy to make trick-or-treat stock ups easier?)
Honestly, I think it would be cool to give them something more to work with, they've all got such neat designs it'd be a delight to see them more!