tfw you go too deep into the theory chamber... here are my predictions!! x_x
Swimming, Diving and the Museum
? Swimming and diving are either currently implemented and unlockable via gameplay, or not implemented yet and will be added in a big free Summer update. (Here's hoping for the former!)
? An art gallery will be introduced to the museum via some form of upgrade. The museum staircase will eventually lead to said art gallery, diving critters will expand on the fish portion.
? And maybe, just maybe, diving hasn't been shown off yet, because this time around there's a whole new layer to it. Maybe you can actually see some form of an underwater environment while diving? Perhaps you can snorkel in shallow waters, and you'll dive New Leaf style into even deeper waters to catch
Gardening and Leif
? Leif's gardening store will be located in either the Nook store, or a botanical garden-like area within the museum.
? It will not only contain the classic items, but all new trees and plants too, not unlike the new greenery you can see all throughout the museum.
Character Customisation
? Harriet returns running a somewhat more general beauty salon. There will be more hairstyles, hair colours, contact colours and wigs to buy from her.
? I also get the feeling we could see perhaps a couple of new nose and mouth types in the future.
? At the very least we will see custom design templates for a few styles of skirts, shorts and pants.
New Neighbours
? We'll see a whopping 100+ new animal islanders just like New Leaf, including all previously absent villagers. (Some receiving name changes and/or design tweaks.)
? 1 or 2 new villager species, along with 2 new personality types to bring it to a total of 10 like your villager cap! (Perhaps something like Shy and Geeky?)
Familiar Faces
? Serena will return with perhaps a similar or slightly enhanced role. (That being obtaining some sort of special tool.) But instead of being summoned via fountain, she's summoned from the ocean.