Don't misunderstand me; plenty of things interest me about the project. It is just that I fear my interest is going to start fading soon if I don't start writing and concretizing these characters and their desires even more. But in the style of vicious cycles, I feel I can't write without knowing more. With my first-ever project, I knew the ending, more or less, but I didn't know any of the in-between, or the beginning for that matter. In this case, I know the beginning and nothing else. Because TTD took me 6 years of wandering around and bumping into corners, I'm loath to repeat the same, "Here I go on a jolly journey of discovery" scenario, as appealing as I find the notion of discovering things at the same time as the characters.
I can whine about what-to-do? what-to-do? but I think the answer is simply: Sit down and submit the story idea to a Writerly Inquistion. The primordial question being: What kind of story do I want to tell?* When I have the answer to that, I can figure out the elements that need to be in there and how many of them are going to be thorny historical problems.
* It isn't as if I haven't already asked this question; I just stopped digging before I got to the correct answer.