So I think I wrote about this else where but hey why not!
New trees for your plaza,
the normal sapling, a cedar tree, maybe a willow tree, a tall redwood.
New events;
playing island games with your villagers.
Go underground in a cave for gems and metals, maybe stumble across Resetti's family.
Mayor options:
house placing, determined by those old little notes shaped as a house in pervious games.
Able to unlock PWPs by researching them or buying "blueprints" for a different number of bells.
Character creation:
I don't want to bring up race, but every game it's only been white audience, time to grow up and allow us to create our own character with different eyes, hair, skin, noses, etc. Maybe even no eyes, like Coco from Gamecube.