As you might expect, Avalon is filled with knights, castles, goblins, dragons, and other assorted medieval tropes. The expansion also "plays heavily on the Arthurian and Celtic mythologies," according to KingsIsle. Additionally, Avalon will allow players to uncover the motivations of Wizard101's main characters (including Merle Ambrose and Morganthe).
The expansion boasts six new zones, 15 new areas, a level cap increase (80), 28 new spells, new gear, pets, and mounts, and the ability to achieve rank 15 in gardening.
[Source: KingsIsle press release]