I want the phone booths to at least open and close and access my closet. Even better if they would dial the psychic hotline like the old Lovely Phones so I could wear a lucky item each day!
I would say the snack and drinking machines. Would be funny if you could spend like 100 bells for a sandwich or a drink which you can carry around like your villager do and even eat/drink it at some point. I'm also disappointed that you can use the pool, like if the villagers at least could use it, then it wouldn't be such a waste of Nook Miles. Interacting with the playground items would be nice too, like come on Nintendo, let me and my villagers use the dang Springy ride-ons.
The signpost items being functional. Similar to Minecraft, you'd be able to tap 'A' beside one, input text, and visitors would then be able to read your signs when visiting. Useful for naming specific areas of your island and helping direct people to them.
One thing I miss from the early AC games was being able to make a wish in the fountain. I'd love to have that interaction with the fountain DIY instead of it being just for show. If they bring back a fully functional luck system, this could play a part in it. Maybe you'd get rewarded for wishing a certain amount of times? A drinking animation for the drinking fountain/garden faucet would be a nice touch as well.