For my v3 campaign, I'm thinking about setting in the aftermath of my v2 game. My group veered WAY off the path in Ashes of Middenheim, and all but one of them ended up deciding that they would just join the Khorne cultists. That campaign came to an end after that, but I think it would be interesting to continue with the setting and have been trying to think of the ramifications of Khorne cultists taking over the Cult of Ulric.
A side note: My Khorne cultists are capable of being devious and planning ahead. I don't really know how Khorne can have a secret anything the way Khorne followers are often portrayed.
The new leadership of the Cult of Ulric starts distributing a lot of negative rhetoric against Sigmar and the southern states. Things like, Ulric is the true God and was around before Sigmar, the northern peoples are the ones that do all the real fighting and deal with all the hardship when Chaos invades, the southerners just show up in time for a couple fights then claim victory, etc. This escalates over the course of a year or so with Chaos cultists continuing to spread hate and incense the populace. With conditions in the north being so bad from the devastation in the wake of the Storm of Chaos, many people in the northern states are swayed by the arguments accusing the south of hoarding food and supplies and looking down on their northern cousins. A large number of people are dissatisfied with rule from Altdorf, and independence of Middenland, Nordland, and Hochland is declared, calling themselves the Teutogen Empire.
Karl Franz obviously isn't too happy about this. He doesn't want to deal with this militarily for a number of reasons: The Empire is weakened after the Chaos incursion, and civil war will greatly exacerbate that. He has released the army he gathered for the Storm of Chaos, and calling everyone back so soon would be terrible for morale. Also, after the harsh winter and low yield of the previous year, the Empire really needs time to cultivate crops and rebuild the agricultural sector. Envoys he has sent to Middenheim continually run into difficulties and often have to turn back.
Now, army patrols are stretched thin trying to keep the peace along the border with MIddenland in addition to an increased presence after the Storm of Chaos and their normal duties. Some Sigmar devotees are fleeing the northlands, and refugee camps have formed on the southern shore of the Reik. Some chapters of the Knights of Ulric and some priests of Ulric have begun to suspect treachery in their own ranks, and have left their homes and temples to find proof of what is going on.
So, that's what I have so far. I like thinking about large scale plot development stuff rather than the small details, so what I usually do is take published adventures and weave them together in my overarching plot.
I have started coming up with some more details, but I thought I would post this overview and see if I could get some feedback suggestions. It might be a little over the top, but I don't think it's a totally unreasonable sequence of events.