Just Throwing it Out There...
So, I anticipate that a basic plot line would suffice, and that would still leave authors with much freedom to contribute to the work. The reason that I stress this is because I have perceived that some individuals wish for complete freedom, and some seem to wish for a larger, more concrete plotline before we begin. As keeping with the ego (just kidding, psychoanalysis is mostly a fallacy, sorry Freud), I believe it would be wise to seek a sort of compromise: a basic plot line that the writers are free to express the work upon. There is no building to construct if the foundation is not first established.
So, for the plot, here is my basic, very general, idea: something that has to do with fantastical, Middle-Age warfare, perhaps? (I would imagine that some degree of cannons and firearms would be permitted, as cannons have been around since the 1300s, and firearms have been around since the early 1400s - technically the Middle Ages). But, in the end, I will likely proceed as a writer with whatever plot gets chosen.