DISCLAIMER: This is MY opinion. Not fact. Nobody but Blizzard knows for sure what is coming.

I believe that we will see one more content patch after the current one coming and that will be the patch that allows us to fight Arthas which in my opinion will come late 2009, or early 2010. After the death of Arthas it will allow for the next chapter to begin the return of Azshara.

The Next Expansion

World of Warcraft: Cataclysm

Location: The South Seas (Kezan, Plunder Isle, Zandalar, Broken Isles, Tomb Of Sargeras..etc)

Plot: Azshara rises from the ocean with her Naga army to destroy/conquer Azeroth. Alliance and Horde after having defeated Arthas regroup and get permission from the Goblins to use The Undermine on the Island of Kezan as a base of operations with which to fight back at Queen Azshara.

New Major City: The Undermine

New Races: Alliance: Possible breakaway faction of Naga
Horde: Possible breakaway faction of Goblin

New Hero Class: Unknown

New Features: Underwater Mounts
Underwater Instances
New Water Forms for Druids
Boat Mounts

Possible Weather Enhancements: Heavy Storms (IE. Hurricanes)

Other possible additions: Gilneas, Kul Tiras