I've had plenty over the last few days so I'll see which ones come to mind off the top of my head since I guess those are the ones that are most pertinent to me. xD
Definitely some way to turn the camera at least a little bit. I don't necessarily want to be able to move it behind my character or be in first person or anything but this would at least make for some nice photo-ops.
I'd like to see animal neighbours be able to be in places like the museum or something but still be able to interact with them fully instead of just having them cycle their specific dialogue to that place. It would make things feel more alive and it'd be less of an annoyance to find out they were there.
It would be cool to have multiple houses without having to have more than one character. I personally never wanted to add more than myself to my towns because it broke the immersion a bit splitting myself between different characters just so I could have extra houses in the town! I think being able to have extra properties would solve this! And it also links to my next point...
Having more than one 'area' to the town. This basically means I would want a bigger town, but in a way that... let's see how to put it... you know how the campsite in the welcome amiibo update is its own area which you access through the side of the cliff? I would want to have multiple whole-town sized areas separated with those load screens that all connected together - you could buy more land for your town to have it as big or as small as you wanted, and this would allow you to have kind of districts to your town without it all being overwhelming all at once!
I have also thought this for a long time - designing animal villagers!! I would love there to at least be a basic editor that allows us to pick colours, species, patterns, personality and the like, so we could even better personalise our villages with our own special animals. It would be so cool to see what people created and be able to move them in and out of others' towns like the existing villagers. Building on this, I don't know how popular this would be, but the option to have a customised animal in place of some of the special animals like shopkeepers and the like might be kinda cool. I think it would be cool if there were different animals running the shops amongst different towns rather than having infinite Tom Nooks. But then, those characters are super beloved so I don't think this would be for everyone! It's just a thought.
I would definitely love to be able to build animals' houses for them. Instead of them just moving wherever they wanted you could receive a request from an animal that they wanted to move in and just like HHD you would have to build their house for them. It makes more sense if you're the owner of the town in question to be able to decide where they're allowed their house! Alternatively you could tell them they could move wherever they wanted and you could skip all this if you didn't really care. It's all about options!
Annnnd one final thing that I also don't think will be popular with everyone. What if when you moved to the town, you had to kind of build it from the ground up? Perhaps there's nobody or very few people living there when you arrive, and you get to build it into a thriving town over time? That would personally appeal to me as I love to feel like the real owner and main influencer of anything I get to be in charge of in creative games. I'm taking a lot of inspiration from Harvest Moon: A New Beginning with these ideas. But I feel like I might be trying to turn Animal Crossing into something it's not meant to be. So these are of course just ideas and I would be happy with any of them not being a part of the newest installment.