After the news that they're co-producing AAIT&S, I wouldn't be at all surprised if Series 8 is announced as a full-on BBC/BBC America co-production (rather than them chucking some money in every now and then as they do currently). That looks like being the only way of keeping up full-length (as we currently understand the term) series.
If so, we'd likely see the split season malarkey go away. From conversations I've had with people at BBC America, they don't like the split seasons because the run's too short and they don't have the marketing budget to hammer two premieres. If it came down to it, I suspect there would be a compromise between Wales and America on a single run of 10 episodes plus a Christmas special.