Have you ever read anything by Stephen R Lawhead? Loved the first three books of the Pendragon cycle (Taliesin, Merlin and Arthur). He wrote a a couple of new ones (Pendragon and Grail) but they came out much later and I felt the original three stood on their own and didn't need anything added. Basically the Arthurian legend set against the backdrop of the fall of the Roman Empire, the rise of Christianity and the end of the druids. He also throws a little Atlantis into it for good measure.
He also did a Robin Hood trilogy (Hood, Scarlet and Tuck) but rooted the character in Wales.
They guy does a lot of research and it shows in his writing, although my wife thinks the religious overtones in some of them can get a little over the top. She did like his take on St. Patrick though. It's called Patrick: Son of Ireland.