The Rhoynar and their struggle against the Valyrian Freehold would be incredible. The Rhoynar had water magicians guys. Dragons against fucking water benders...HOLY SHIT! Story starts with the Rhoynar allowing Valerian settlers into their lands. Settlers who eventually decide that they'd rather take those lands for themselves. The Rhoynar fight back, and eventually one of their kings puts together the largest army ever assembled, and goes ham on these Valerian dick heads. With their badass water bending skills to protect them somewhat from dragon fire, the Rhoynarian King and his forces defeat a Valarian Freehold Army that consisted of dozens of dragons.
Old Valyria isn't all to happy with these developments. So Mother Valyria marshals a force that included hundreds of dragons, and decide that simply subduing the Rhoynar wasn't enough; the complete and utter annihilation of their civilisation was the only appropriate response. The brave king is defeated in a battle, and is held in chains and made to witness as mighty Valyria lay waste every single Rhoynar city they capture. Valyria has some how learned a way to nullify the great water magic that had aided their enemy for so long.
In desperation, a young Rhoynar Queen named Nymeria rallies what's left of her people, and begins the process of organising the largest migration out of Essos ever attempted. She manages to land 10,000 Rhonyr ships onto the shores of Dorne in Westeros. She had the ships burned to symbolise that there home was now in this new land they've arrived on. Dorne is divided into many kingdoms at this point, but it's the Martells that the Queen decides to ally with. Swelled with an army of battle experienced Rhoynar and a marriage pact to their queen, the Martells bring all of Dorne beneath their banner (a banner that now has the sun emblem of the Rhaynor). The new Dornish Queen however, would forever change the socio-political environment in her new lands, by doing away with the laws of the Andals, and replacing them with the customs of her people. From this point forward, the eldest child regardless of sex, will inherit their parent's titles.
Give me this HBO. I don't care about Robert's Rebellion, or the Dance of Dragons, or Aegon's Landing. Give me Dragons versus water magic.