My first vote is stormlands. The infighting between Renly, Stannis and the pretender Joffery would be too good to pass up. Second vote would be riverlands.
The north is way overdone in my taste. It would be good but just over used. The riverlands and stormlands would be the best bet in my opinion. More division and open to a lot more sub plots. I'm personal interest resides with the stormlands personally.
Dorne would be interesting if it was around the time of Joffery's wedding and the duel between Gregor and Obeyren.
I'm not new to the site just back from an extraordinarily long hiatus. I can't remember my old password/email so I've made a new and hopefully, I can get back in the swing of things without too much trouble.