As, um, someone who regards Yuyuko in far too well a light to be sane, I have to ask: does this include pre-death or is it just Ghost Princess Yuyuko? Because while I kinda like mortal Yuyuko stuff, we pretty much know how that ends - her sacrifice ends the specter of her terrible power and seals the Saigyou Ayakashi from eating all the souls ever. Yukari and a Konpaku were around, and probably helped out. There may have been others involved as well but there is no evidence of them; a pity because this is the kind of stuff shrine maidens specialize in. So it puts some constraints on the kinds of ideas that can be used.
But placing it fully between death and PCB opens up all kinds of freedom to write your own plot lines. There are 1000 years in there, and a lot of characters and ideas to mess around with. Did Yuyuko lose her memory slowly or all at once? How would she react to a young Aya or Ran? What sort of trouble could there be between Hakugyokurou and Higan? Did Yuyuko play a role in the whole Hakurei Barrier/Underground Youkai thing? The ever-popular "Where did Youki go?" And, of course, anything else you can think up! As long as it contains plenty of Yuyuko, and doesn't directly contradict the knowns, it's all good.
*can no longer resist and points in a frothingly maniacal manner at Yukari*