What I think would be good is that the the Able Sisters shop would be slightly bigger (6 shirts/pant/shoes per day, with 4 hats/accessories and 2 umbrellas) and that the designs you see Sable making actually went on sale the next day. A DLC idea for their shop would be that when you wake up one day and Mable is at your door, telling you that they have exclusive patterns available for the week for free and would replace the normal free patterns they have, and would revert back to normal after the week passed.
I also kinda like the idea of having your own swimming pool and garden which each have loads of different designs, but they cost extra when you upgrade your house.
Oh and maybe Shampoodle could start up a massage service (which costs like 500 bells) and it protects you against bee stings for 1 or 2 days? But it might be a bit too RPG-ish.