After trying something different with New Horizons, I have a feeling the next game will be more of a "traditional" Animal Crossing game. So I could see us moving into an existing town, not starting on a deserted island from scratch. I wouldn't be surprised if crafting is gone, but the town-designing abilities
need to return. I can't imagine returning to the old way: "Oh, this bend in the river is one space too short, so I'll
never be able to add a bridge here." Once you introduce a colossal designing feature that's integral to town development like that, there's no going back IMO.
There will need to be some sort of city or Main Street, with a variety of shops (hopefully actual stores and not just vendors offering a few items each). Maybe there could even be a city where you can run into other players? (Like Club Tortimer, but in a city/shopping setting?) That would be a good excuse to bring back the auction house from City Folk, where we could sell and bid on other players' items.
I doubt the game will be on Switch, though. Most likely, it'll be on the next system...and probably several years away. But of course, Nintendo is hard to predict (especially when it comes to Animal Crossing)...so who knows? With ACNH being the 2nd-best selling Switch game, I hope Nintendo will see the value of the series and not make us wait seven years between games again.