I've seen a few things lately denigrating horses-and-castles fantasy, anything set in medieval or pseudo-Dark Ages. Of course, my new idea, Sundered Sword, needs a historical milieu.
The idea started with the line from Monty Python and the Holy Grail. "[S]trange women lyin' in ponds distributin' swords is no basis for a system of government." Which got me thinking about Excalibur and wondering what would happen if two different people both got swords -- or part of the same sword.
I can't write about such a thing in a modern world or even post-Enlightenment. People don't believe in divine right to rule and haven't for centuries. That pretty much limits the sort of fantastical world I can create.
It won't be a strictly medieval world. I've been reading a lovely biography of a woman in early seventeenth-century Italy, and I'll be borrowing Renaissance and Baroque elements, probably including guns and cannon. But the absolutely critical point is at heart, people must believe in the right of kings (or queens) to rule, even if it has been generations since one has.
And that's my rationale for adding yet another horses-and-swords fantasy to the world.