Things that could be added to Diagon Alley, list from Potter Wiki.
Eeylops Owl Emporium - Takes away heavy crowd from other Owlery
Florean Fortescue's Ice Cream Parlour - We don't have.
Flourish and Blotts - Sell all things literature of Potter here.
Gringotts Wizarding Bank - Ride maybe?
Junk shop - Discounted merchandise on all things Potter.
The Leaky Cauldron - Because it's a Pub, so yeah.
Madam Malkin's Robes for All Occasions - Clothes to buy
Madam Primpernelle's Beautifying Potions - Create your own sodas?
Magical Menagerie: More merch themed to the monsters & creatures of the WW.
Ollivanders - An obvious choice, needs to be a lot bigger.
Sugarplum's Sweets Shop: A shop that sells confections.
TerrorTours: Depedning on scope of expansion, maybe have a tour to show you throughout the WW. Or, book tours for future UO trips.
Weasleys' Wizard Wheezes - Joke shop
Wiseacre's Wizarding Equipment: A miscellaneous equipment store.
Various stalls and peddlers: Chestnuts, Butterbeer, etc.
I think it's different enough, yet also has elements to take away the craziness of some of the Hogsmeade shops.