The regions I am particularly interested in;

the [b]North[/b] - So much happens here in this time period, with plenty of betrayal and divisions. Also thematically very pleasing. A bit overdone though.
the [b]Riverlands[/b] - A neglected Kingdom in some regards. Is sacked once in this time frame, and invaded soon after.
the [b]Stormlands[/b] - Perhaps divided between Renly/Stannis in the war. A unique region in it's loyalties.

I'm also open to any other kingdom, though. These are just off the top of my head what appeal most.