Frogboy can't share what's coming in the expansion pack so I will take a crack at it...
There are certain things this game desperately needs many of which you can see just from reading the forums... Interestingly, SD has publicly committed to many of them, but has been mum on the others...hmmmmmmm.
Therefore, the expansion pack must contain:
1. Improved hero/champion system. There will be more heroes and they will have more interesting and upgradeable skills and attributes. This will apply to the sovereigns as well.
2. Lots more quests. There will be more random quests and better quest tracking capabilities.
3. More notable locations. There will be lots more notable locations including dungeon crawls.
4. Uncommon/Rare/Unique item drops. You get the idea.
5. Interactive landscape. We'll see active volcanoes, forests that can be cut down and harvested, rain/snow, etc.
6. More customizable troop options. Yes, you'll finally get to create a gaggle of blond archers in pink dresses!
7. Events. You'll be able turn on/off or mod random events.
8. Many UI improvements. City screens, diplomacy overviews, interactive kingdom management dashboards, etc.
9. Multi-player options. Hotseat and PBEM.
10. And finally ... BEAR FREAKIN CALVARY!!!