Better AI for the villagers, more areas outside of your village to explore--a larger game world would be easy on Switch--and more customisation over your village would be grand.

That's a great idea, makes me think of pokemon diamond and pearl!

Different biomes to travel to with trees, animals, and plants to match. For example: A desert area, a jungle area, a marshland, a large metro, tundra, and mountains. Maybe have the player choose which type of land they'd like their village to be in. They can then travel to other player's worlds to collect things that they couldn't normally collect in their own world.

I agree, and also battles, and maybe a villain too. All towns of people you know being placed on the world map.

But how would you defeat the villian? AC is such a non-violent series. 

Bokujo Monogatari (Harvest Moon from 97 to 2013) used to be that way too, then they added battles for the Rune Factory spinoff.

I think a lot of AC fans would be turned off by any sort of battles. AC is hugely popular amoung girls because, a lot of what you do in AC fits within traditional roles for women. If battles were a required section of gameplay I don't think a lot of the women, or casuals would bother. 

Mecha

Roommates would be cool. Bigger Everything. Having different terrain and cultural elements from those different areas would be amazing. Forget town fruit, you could have to go to far off places to get whole sets of clothes, etc. They could have a trader come in with a few items too though. More abilities to build onto the town and give people resources. You could see neighbors putting in add-ons to the houses to talk about and explore. More varied trees and plants and continue adding town beautification items like they did with the fountain, street lamps, etc. More interactivity. Throwing a house party and determining what to serve. Give more sim functionality, but in an animal crossing way. More definition for more art creation options - dumbed down photoshop. Curve creation tools and more - along with an approximation art tool, where the tailor will give several versions of what you were trying to create as suggestions (for those that art suck).

change the date or speed up time in animal crossing, like Plague inc