All blizzard has to do it take 1 single unused concept, and turn it into a whole expansion.
- What lay beyond the dark portal
- Arthas and the scourge
- Deathwing
- Pandaren
They can take anything and throw in endless content to work with it.
Azshara:
- Naga/murloc races
- New water puking class
- undersea area where we can live and breath with some happy fish men
- lots of old night elf resources we can pilfer
Argus:
- A neutral half demon race that joins with the horde/alliance later
- Argus and all the lands as it become corrupted by the legion