For me it would be:
Being able to have one villager live on the island, like in the old games.
Being able to destroy rocks permanently...or at least when you hit ANY rock (not just gem rocks) with the shovel it will bust apart. I find rocks annoying because they interfere with the flow and design of the town, especially if you want to put down a project somewhere or want to decorate with patterns.
Being able to drop things down onto patterns. It's kind of annoying that you can't do this. If you shake a tree and there's no pattern-free space under it the fruit just disappears into oblivion or something... Or if you pop a balloon over an area with patterns the present just vanishes...I do not like this.
Perhaps fertilizer for grass deterioration?
More space for patterns. Frankly I'm really surprised they didn't decide to give us more room for patterns? Even though they introduced the QR machine in this game? Like....what the heck. Obviously we'd need more space for patterns now more than ever in this game!
A little more freedom with placement of public works...It can be kind of restricting at times.
Shrubs being able to be grown around houses. From what I've heard shurbs are pretty darn finicky!
Personal opinion but I kind of miss the design of the map/world in earlier AC games...I don't particularly like the sudden cliff and then the beach underneath that. For example I liked when ramps did this:
1,
2 ,
3 ,
4
I just liked when the beach was a gradual slope and not a cliff over looking the beach.
They should also bring forked rivers back! Because:
Mini islands!!
Hmm that's all I can think of atm. I might edit later!