Action RPGs such as Dungeon Siege III are ripe for expansion packs and downloadable content due to their easily extendable gameplay. Previous iterations in the series saw retail-based expansions offering considerable amounts of new gameplay, but we live in the downloadable age now, so it's likely we'll see new content in smaller, more regular spurts.
That said, the upcoming Treasures of the Sun DLC for Dungeon Siege III sounds like it includes a fair amount of new content. It bumps up the game's level cap from 30 to 35, for one thing; it also adds new abilities; new monsters; new places to explore; new loot to find; and also allows players to respec their talent and proficiency points. It also includes a new Enchanting system where transmuting items gives players the chance to acquire Vials for weapon and armor enhancement.
Treasures of the Sun will be available in October for PC via Steam, PS3 and Xbox for $9.99 (or 800 Microsoft Points, presumably).