I enjoy your work tremendously, and also appreciate your care in adding historical notes at the end. (The only book of yours that did not work for me was your work about Stone Henge). I trust you to write a good tale, especially one with historical roots.
Therefore what I'm about to say may sound strange, as I know history has limited applicability. But you've done such a wonderful with the Grail series, etc, that I'd like to suggest, not a story, not a plot, but a subject: Robin Hood. That should avoid any of your mentioned problems with lawyers, I dare hope. It is a subject I dearly loved growing up, and to judge by that truly bad and inaccurate 2010 film with Russell Crowe, the topic still has commercial appeal. I realize you're probably not struggling desperately for writing ideas/topics, but as a fan, I would be interested in seeing what life you can breathe into such an incredibly rich character (and his companions, of course). Other than Arthur, what greater figure of British folklore is there?
Just a wish, hoping I strike a cord, or bowstring, as the case may be.
Ralph