Definitely the Dream Suite, that is just such a fantastic feature! In fact I'd love for them to expand on that in some way, maybe make it possible to visit a dream suite together with a friend or something. That'd be really cool.
Public Works Projects should be like it is in New Leaf, but expanded a bit such as having more types of projects.
Also Tortimer Island was great as it allowed you to catch high-value Fish and Bugs even when it is not Summer or Winter.
In City Folk. In order to farm bells, I'd have to go for Sharks and Dorado's (Both 15,000) in Summer, Stringfish (15,000) and Tuna's (7,000) in Winter (Oarfish didn't exist in CF) and Barred Knifejaw's (5,000) for any other season.
Also I think having Main Street was a good call. They should keep that instead of having the individual buildings all over the town like in City Folk.
Also, the large amount of Nook expansions in New leaf was good too. You only got a second floor and Basement in CF.
Hoping the Dream Suite and Harvey’s campground come back for sure! Another thing I hope for would be the Happy Home Showcase to return. It was a lot of fun to visit Reggie’s and the other Nintendo staff houses. I think it can be expanded even more so I hope it comes back.