Well there is the always-changing map at the beginning of each episode.
After reading your questions I just did a quick thought on who the most powerful families in Westeros (post-Targaryen) are. And from what I've read in the first four books, I'd rank them:
1. Lannisters (Westerlands)
2. Tyrells (The Reach)
3. Baratheons (Stormlands)
4. Starks (North)
5. Martells (Dorne)
6. Tullys (Riverlands)
7. Arryns (Vale)
8. Freys (Riverlands)
9. Greyjoys (Iron Islands/North)
10. Boltons (North)
All of them will play big roles. I actually could see the show downsizing the Martell's part in the story as it is kind of marginalized, but at least the Red Viper (a Prince of Dorne) will play a major role in the fourth season. Hope that helps.