Here are some of my ideas:
Room of Requirement (scene from Deathly Hallows pt 2)
Price: £79.99
Minifigs: Harry Potter (casual), Ron Weasley (casual), Hermione Granger (casual), Draco Malfoy (casual), Gregory Goyle (uniform) and Blaise Zabini (uniform).
The set would come with Ravenclaw's Diadem, lots of junk from old students, the Vanishing Cabinet and lots of fire.
Whomping Willow (scene from Prisoner of Azkaban)
Price: £39.99
Minifigs: Harry Potter (casual), Ron Weasley (casual), Hermione Granger (casual), Sirius Black (prison clothes), the Grim and Scabbers.
The set would come with the Whomping Willow with hole at bottom for the Grim to take Ron into and some random plants.
The Great Gringotts Escape (scene from Deathly Hallows part 2)
Price: £99.99
Minifigs: Harry Potter (casual with Invisiblity Cloak), Ron Weasley (Death Eater disguise, double sided head), Hermione Granger (Bellatrix Lestrange disguise, double sided head), Griphook, Travers (double sided head, one normal, one Imperiused), 2 Gringotts Goblins, 2 Gringotts Wizard Guards and the Dragon.
The set would come with Gringotts Bank with exploding function, Gringotts Main Hall, Bellatrix Lestrange's vault, Hufflepuff's cup, Sword of Godric Gryffindor, some coins, some other treasure for the vault, waterfall that removes any cloaking enchantments, large chains and a brown 64x64 baseplate to put everything onto.
Lovegood's House (scene from Deathly Hallows part 1)
Price: £59.99
Minifigs: Harry Potter (casual), Hermione Granger (casual), Ron Weasley (casual), Xenophilius Lovegood, Travers and Yaxley.
The set would come with Lovegood's House with exploding function, Xenophilius' Dirigible Plum Garden, model of Ravenclaw's Diadem, Erumpent Horn, printed brown book (The Tales of Beedle the Bard (with page with story of The Three Brothers printed inside)) and some random plants.
Lord Embo