Here is where I see some of it going for Cataclysm.

Orcs - The original story of the orcs was trying to find a place in the world. All the quests kind of represent clearing out the rest of Durotar. Now, the story seems like it will shift more to the fact the Horde is grounded, stable, and strong. The story of Garrosh will instead be the driving point, the animosity of his ascension, the pilgrimage (and return) of Thrall, basically the building of an internal civil war. We will find many orcs happy, and many unhappy, with the events of the time.

Tauren - With the death (or absence) of Cairne, Magatha will make her move to claim power over all the tribes. However, her attempt will be thwarted by Baine, who will take the place of his father as the Chieften. Baine will banish Magatha from Thunderbluff, forcing her to find sanctuary somewhere else. She ends up finding that place in Orgrimmar. Garrosh makes her his advisor, and her Grimtotem become entrenched with the other defense forces of the city. This strains the relations between Thunderbluff and Orgrimmar, but the two remain allies out of necessity, with the Alliance literally pushing it's way to the doorstep of Mulgore.

Trolls - They never had a story, but this will be the best time to really start giving them one. Expand Sen'jin and make it, at the very least, a regular town. Vol'jin will be leading from the city, and will be one of Garrosh's main dissidents after being kicked out of the city in the interests of appointing Magatha. Due to the unrest, the Trolls and Tauren (Thunderbluff) will grow stronger, with places like Crossroads gaining a much larger troll presence. They still ally with Garrosh out of loyalty to Thrall, hoping that he will set things right when he returns.

Not really sure on the others just this yet.