How "fantastical" do we want the saga to be?
Do we want a "low fantasy", more realistic saga (like the Song of Ice and Fire or that atrocious Merlin movie that came out a few years ago)?
Or do we want a more "high fantasy", along the lines of old-school D&D, with griffins and dragons and merfolk and all sorts of weirdness around every bend of the road?
Or, more likely, something in the middle, a la Harry Potter? And, if so, where on the scale do we want?
And, in a different vein, how "hard-core" do we want the saga? GoT, where people get killed and very bad things can happen, or more Potter-esque, where things look bad but everything turns out in the end?