Blizzard have not really discussed any expansion plans for Diablo 3 yet, which isn’t really surprising considering that the main game isn’t even in Alpha stage yet. The same could be said about StarCraft II up until BlizzCon 2008. Blizzard had mentioned nothing of additional expansions of the game before October.
With StarCraft II, the team have created a plethora of lore and content for the game, and they came to the conclusion that squeezing this down to 10 missions for each of the three races would seriously hamper the epic scope of the new RTS. As a bit of a background to this; they had originally planned two expansions for the game, much like Brood Wars, just adding a second one on top of it. Instead of squeezing the game down, or pushing the production time up another 2 years, they decided to push the story contents out over the two planned expansions, and hence they announced the StarCraft Trilogy at BlizzCon. They would most likely not have revealed their plans for having two expansions until well after release if this had not come to pass.
As Blizzard is now looking to release content on a regular basis, like World of WarCraft and “one expansion per year” goal, it would be UNLIKELY that they would do just one (or no) expansion to Diablo 3. They have already hinted that the story line will tie up loose threads, and make it possible to add new content, be it games, expansions, books or comics to the Diablo Universe.
Now, MTV talked to Mike Morhaime about World of WarCraft expansions, and he said:
Hmmm, is there more profit? In an ideal scenario, I think we’d probably do [expansions] a little bit more often than we’ve been able to do. But I agree — I think putting too many expansions out too frequently, you probably get to the point where people do need to start taking a little break.