AppleCracker
Call me Mikey
Lol

you, my friend, are a genius. That would be amazing.TREEEEEHOUSE! with the option to customise!
Tree house would be sooo cuteI would like to build more park stuff like a see saw, merry go round, swing set, and to be able to see the villagers playing on it!!! Apple would look so cute ;-; also something cool would be garden light (short lamps) that you can line paths with. Perhaps PWPS that looks like flowers but are things like benches or lamps (sort of like weeding day set). Also it would be cool to have starry/space themed renovations for town hall and train station.
I was going to suggest a bakery shop connected to Brewster's cafe shop (as if it was an optional upgrade or something), but it looks like a lot of people had the same idea. Same goes with the playground idea.
Food wise though, I wouldn't mind a grocery store stocked with fruits/veggies, bakery needs (which includes utensils), and spice. AC already have a couple of food items players can eat/consume, ingredients to gather during Harvest Festival to make dishes. I was very disappointed not being able to make coffee with the coffee beans and coffee maker Brewster gave me (I want to pretend cook, lol, but hey, new table toppers for kitchen based rooms). Maybe Leif could run that shop, or a new character could run it.
I wouldn't mind a cave though. Those permanent rocks will still be there (for money rock reasons), but the cave could add toward the mysterious spawning gem-rock's existence. May help speed up the process in gathering certain color ores. Cave could be located on the wall/end side (non-ocean side) of the map, or it locating behind the waterfall. If you walk in the tunnel within the cave far/deep enough, you might come across Mr. Resetti (that manhole thing is so weird).
We could also have an upgrade to Club LoL, which will introduce us to the arcade (or just a carnival/circus-ish play world). We have mini-games in AC, a claw machine, and other random novelty knit-knacks (balloons, pinwheels, etc), I wouldn't mind having a place to play those games with friends. The makers of AC could also add a monthly/bi-monthly game competition, the reward for winning will give you the actual gaming system furniture item to place in your house. Dr Shrunk's wife will manage that area, maybe.