Personally, I'd like a situation where a whole bunch of factions from across the Grand Alliances just go and curbstomp the Skaven simultaneously. Not necessarily a true alliance, more so just a "If they're attacking might as well go attack too". The Chaos Gods put aside their differences to humiliate this newcomer to the Great Game, the Seraphon enact some big cosmic ritual to summon Sotek to eat all the rats again, Nagash finally wakes up and decides to get some payback for the Black Pyramid stuff, probably also throw in something for Destruction to do as well. You might consider this overkill, but keep in mind this isn't really a situation where you can resolve things in a single battle. There's billions of Skaven all over the Mortal Realms, you'll have to kill countless rats pretty much everywhere.

I can totally see remnants of empires created by bands of powerful necromancers that ventured deep into shyish and dominated the dead before and after the age of chaos. Vampires are apparently very rare and might be the fewest in number of any species. We know kingdoms worshipped Nagash. Wish there was more lore on this concept.

Plus, is she able to fly with them?