Like everyone else has said... more stuff to do online. I rarely ever open my island anymore (even less since they added Dreams back into the game), because there isn't anything to do online... especially anything you can't do while visiting an island via the Dreams... besides shopping, but then again someone can just easily mail you something or you can just wait for that item yourself.. or just visit and then leave. It really makes the game less about playing with other people, and more about the economy in the game.
I miss being able to go to Tortimer's island.. hell even the Bug Off/Fishing Tournys seem like downgrades to me. The Fishing Tourney is less of a tournament, and more of a smol minigame. There isn't a real winner. At least in previous games Chip would give only ONE Gold/Silver/Bronze trophy away each tourney. And you usually had to work to win the tourney. Kept the player coming back to the game that day just so they could win a trophy that week. And New Leaf even had a ceremony that happened from 1800 (6 PM) till 2000 (8 PM) on the day of the tourney (if you won a trophy and couldn't make it for the AWARDS CEREMONY then he would mail you the trophy), but at least you had to somewhat work for said trophy.
I would also like more things to farm, and the ability to separate my clothing storage from my home storage. Like make my clothing have its own storage that doesn't take up any space in my home storage. I have 1,600 slots for everything, but a good chunk of it is taken up by clothing alone... I wanna be able to store furniture, rugs, carpets, and wallpaper, but right now I can't really store more than five of each of those, because I have SO many clothing and materials in my storage. So let us buy upgrades to our storage that max out at say 10,000. I think 10,000 storage slots is more than enough. Hell I think even 5,000 MAYBE 8,000 is plenty as well, and along with the separate clothing storage.
Also let us order more than five items a day from the catalog... I really miss how in previous games there wasn't a limit on how much one can order from the catalog. I hate how I have to balance out the orders for my other characters with the orders for my main character. (I order from one catalog on my main character, and then just gift the items to my other characters).
I also wanna see the post office return or at least some way to have our orders be delivered twice DAILY. 0900 (9 AM) and 1700 (5 PM). That's how it was in the previous games. It makes less sense how this game is more restrictive with time than previous games, when previous games had less for one to do with the furniture they purchased each day. A lot of people can make quick minor improvements to their islands if the mail came twice every day. I also miss talking to Pete, and hearing about how he was doing and his thoughts towards the pelicans running the post office.
I wanna see Leif be able to get his own shop at some point. It makes sense that he'd start out as a traveling merchant. I think a lot of people are forgetting that our islands start out as deserted islands. HELL WE HELPED BUILD NOOK'S CRANNY! You expect me to believe that the other merchants should just magically want a shop on YOUR island?! I think it is fun to see them want to build a shop on your island after you prove your island's worth to said merchant.
For Leif it could be if you have planted, watered, and grown X-amount of hybrids, then he will see that there is promise on your island, and that you and the other residents care about nature, which then makes him feel a kinship with you and your islanders... which of course leads him to wanting a shop on your island.
For Kicks it could be the amount of footware and backpacks you purchase from him. He then sees a market for the stuff on your island, and says that he made a deal with The Able Sisters, and asks if you want him to team up with them to help them sell more stuff.
Lable would be similar, but it would be with how well your character is dressed, and how close you are with her sisters.
Gracie can return, but instead of her car from the GameCube Animal Crossing (yes she was a traveling character back in the first Animal Crossing.. hell she was even a traveling character in Wild World.) She has a boat that she uses to sell her wares. If the player purchases enough of her clothes, and is fashionable enough, then she teams up with the Nooklings and sets up a shop on the second floor of Nook's Cranny.
I would like to see Hariette make a return, even though I probably will never use any of the hair styles/colours she brings, but I do think it would be awesome to see her make a return in some form... Hell they can even have Mii Masks make a return through her. I maybe in the minority with this, but I did like the Mii Masks in City Folk and New Leaf, and I would love to see them return.
Maybe the way you get her to return would be by changing your hairstyle up enough times with a mirror, and showing off your desire for new hair styles... then she visits your island one day, and asks if you are willing to test out one of the new hair styles/colours she came up with. After she visits 3 times, then on the 4th time she asks if you want her to set up a shop somewhere on your island.
Katrina can just be a traveling merchant. I liked it better that way.
Roost should return, but I'd prefer it to be a separate building.
Other than what I listed above...
I would love to have the ability to bulk buy NMT from the machine (i rarely ever purchase them, but it would be nice if I ever need to go to a ton of NMT Islands), and to bulk craft stuff. Hell I would love to have access to the materials in my home storage WHILE I AM IN MY HOUSE! I don't want to randomly have access to that stuff WHILE OUTSIDE.. it would make 0 sense to me. Sure it would be handy, but it would make less sense. In the most I want to pull from my materials from my home storage while I am in my house while crafting.
I would also LOVE to see Desert Island Escape make a return. That game was fun in both Amiibo Festival and New Leaf's Welcome Amiibo. Would be awesome to play that game on the Switch now though. Hell maybe they can make a multiplayer minigame that works with the Nintendo Switch furniture items.. And that could be how you unlock Nintendo-themed furniture. By doing well in those games.. or at least that can be ONE of the ways to unlock those furniture items.
Fortune Cookies should make a return, but WITHOUT microtransactions (i saw someone mention fortune cookies before on this thread, but I wanted to clarify that they should NOT bring Microtransactions with them). You can only buy 1 or 2 a day (works just like the candy does right now), and you get a random fortune, and it allows you to randomly get either a standard furniture item (from a dud fortune), or a Nintendo item (from a good fortune).
The ability to change the design on the grass. I miss the other grass patterns, and I hope we can change those patterns from triangles to either; circles or stars.
They should also add a DIY recipe for the Instrument Shelter item (a PWP from New Leaf that let the player see a weather forcast for that day any time).
https://animalcrossing.fandom.com/wiki/Public_Works_Projects?file=PWP-InstrumentShelter.png
Other than that... I can't think of anything else I'd like to see.