I think it'd be kind of neat to have some level of backward compatibility with New Leaf. Maybe you could visit your old town, move items over, maybe have villagers move in from your old town.
I would like to be able to have more customization when you create your character.
More choices for the shape of your eyes or no eyes (Gyroid eyes)
Choose the shape of you nose, default, round, square shape, or flat (no nose)
Skin color: pale, peach, beige, tan, brown, dark brown, purple, green, blue, or gyroid skin texture.
The shape of your mouth: Default, straight mouth, frowny mouth, goofy smirk mouth, slightly open mouth with small teeth showing a tiny bit, or thin lips that you may choose to be red, pink, soft baby orange color, or a gyroid mouth hole.
If they do create another main AC game (and let's face it, they'd be stupid not to), I would love to see bigger towns, because the towns now are so small and a bit hard to work with, more pocket slots, more convenient ways to move furniture, being able to stack items besides fruit, etc, etc. Would love to see more PWPs added in and be able to put things on paths instead of having to put them around the plaza, train station, etc. More design slots would be nice and I can think of some other stuff as well, but I'm not really sure what all they would add in.