I was thinking about AC, and just realized the strongest system it was ever on was the Wii. Now, I'm not an expert, but the Switch is a tad bit stronger than the Wii, right?
Anyway, I believe it's the series with most potential out of any other franchise from Nintendo (I'd say Pokemon but we know how Gamefreak can't do shit probably will take them eighty years to reach 2012). It has a casual nature to it, but unlike a lot of series it benefits from the mellow tone it carries with it. Even with a small environment, I always got a lot of hours out of every entry in the series.
New Leaf however was different than the first two entries. Now, that isn't to say it was bad (it was spectacular), but it felt more modern compared to the original entries which focused mainly on a woods-like lifestyle. With the Switch, I believe they should combine the best of both worlds and create a game where you can be more than just a mayor for one small town.
If there's anything Nintendo is good at, it's innovation. We don't need an open world AC. Instead, I believe having multiple regions that you can visit and control would open a lot of experiences and possibilities. They can have the original woods that we're accustomed with, and perhaps have a small city in Japan that you can visit and eventually become emperor after doing enough. There are a lot of possibilities - a New York city, arctic cold, islands (wide variety), and a lot more.
Besides implementing more into the game, I think simply expanding the world would be fascinating to see.