we've seen
Tom Nook
Nooklings
Isabelle (joins with Town Hall)
KK Slider
Mabel & Abel (joines with their store)
Label (visitor)
Kicks (visitor)
Wisp (late night, probaby visitor)
Harvey (visitor, eventually heads Photopia)
Blathers (museum)
Celeste (visitor)
Saharah (visitor)
Gulliver (visitor)
Pave, Franklin, Jack, Zipper T and Jingle will all be added with their holidays
Resetti (& presumably his brother) will at least cameo as the rescue squad, I assume you get to see them elswhere at some point.
Joan, Chip & Nat are seemingly replaed by Daisy Mae, CJ & Flick but I can see them being cycled in eventually.
Pete, Phyllis & Pelly are seemingly absent and the Dodos take their place for mail purposes. I could, however, see a proper post office added later on as an option
Harriet's job is in theory replaced by a combination of the Mirror items and just buying hair styles with nook miles. But I can still see her getting either the visitor or shop treatment with fancier hairstyles & colors.
Cyrus & Reese are in theory replaced by Nook and the crafting system, but it would be very easy to make them special visitors featuring special crafts (such as with gems). The ability to treat it as a pawnshop is also unique to Retail, so I can see that being a shop you can still unlock
Unaccounted for:
Katrina (probably still a visitor)
Wendell (may also be a visitor again, or could be tied to the dream suite if it returns)
Luna (Dream towns were relatively popular, I could see the Suite returning as an optional shop)
Copper (Was tied to the optional Police Station with Booker, could see returning)
Booker
Shrunk (Likely back, possibly as a visitor? We don't know how we learn more emotions iirc)
Porter (probably gone, no train station. could see the return of champ?)
Tortimer (unknown)
Brewster (Cafe is probably back as either a standalone shop you can build or an expansion to the museum)
Digby (unknown)
Gracie (visitor status feels replaced by Label, but she could have her own wares and see her high end shop added as a standalone optoin or final upgrade ala New Leaf)
Leif (Likely back as standalone shop/add on to nook shop with expanded flower wares)
Lottie (unknown)
Lyle (unknown)
Redd (probably still a visistor)
Rover (could just be a cameo character, could be on yoru plane, could show up as a random visitor...)
Blanca (likely still tied to April Fools, could be a visitor otherwise)
Katie (could still be a visitor)
Pascal (could still be a visitor)
Phineas (who knows, visitor, badge propieter, something new...)
I could see any one who doesnt make the base game added in the future to add some spice, or to go with new features.