The main thing I want is more holiday events throughout the year akin to Pocket Camps never ending holidays. Add a week long seasonal flower event, gyroidite like hide n seek, make the fortune cookies into an event, and have a week of out of town villagers having a campervan festival in an acre.
Then have one of these each month, and everyones town has a different one for that week, or the same event but different gyroidite, or different fortune cookie items for the week to encourage visiting each other. One thing with the past games is all the holidays are over the winter and then you have a long summer with just the standard single item from Isabelle or bug-off events and that's usually the time of year I stop playing AC because of the lack of different events. PC just fires millions at you all the time and it's kept me engaged, but I think it's probably too frequent for the main series.
Also I want transparency in patterns, mostly for floor tiles. I'd love to have a path that changed with the seasons but just having transparency show the grass through the tile. And this'd make diagonal paths easier.
Some more interaction with Public works from the villagers, and there not to be such a small limit on them. 30 was really low if you wanted anything like streetlights dotted around. I think have larger objects be public works, and then smaller things like benches be furniture that can go outdoors or something. I know previous games couldn't handle that without lagging, but that limitation should be gone on switch.
I'd add Happy Home Designer's way of designing/decorating your house!!
This was actually added in the Welcome Amiibo update. Although a limited version of it to fit in with how NL functioned, and missing the new things to HHD like half tiles/doors/curtains/ceiling items.