I’m a bit iffy on Chaos Undivided, as the current alliance the Pactsworn have are due to the similarities of the Skaven and the forces of Nurgle. Skaven and Slaanesh forces, for example, wouldn’t really have reason to team up.
However, I definitely agree that Daemons of Nurgle should be added as an enemy type at some point, most likely as it’s own expansion just like the Beastmen.
The Bretonnian Civil War, whilst interesting, would be difficult to merge into the overarching story, as the Vampires wouldn’t have any reason to ally with the Pactsworn.
If we’re getting Undead as an enemy type at any point, it would have to be either a Vampire Count or Necromancer throwing in their lot with Chaos (which, in the case of the Vampire, is unlikely).
I highly doubt we’re going to take part in any of the major events of the End Times. Vermintide has mostly only concerned itself with smaller-scale events, and I’d rather prefer it stays at that honestly.
I’d honestly rather have co-op survival against Greenskins be it’s own stand alone game. While yes, any Orc can be convinced to fight against your foe with the right tone, as Malekith managed to convince Grimgor to fight against Archaon during the Battle of Middenheim, Greenskins are generally of the opinion that Chaos or “Da spiky boyz” put up a better fight than “Da humies”, and they often find Skaven untrustworthy even by their own Orc standards, so the chances of them allying with the Pactsworn, whilst a possibility, seems unlikely to me.
Plus, Greenskins on their own have so many units that Fatshark wouldn’t be able to fit all of them in for a single DLC. A separate game where the Greenskin roster could be explored much further is way more preferable, in my opinion.
While yes, Nurgle generally has a distaste of those trying to defy death, he has on occassion, as you’ve pointed out yourself, offered his power to the Undead or at least those with ties to the Undead, such as Heinrich Kemmler, Settra the Imperishable, and Isabella von Carstein (though, she was possessed by a Nurgle Daemon at the time), so the chance of a Necromancer allying with the Pactsworn is not impossible.
The heroes doing a bit of travelling is fine (and I encourage it!), but I feel it has to be within a feasible distance. Anything beyond the Old World seems too far away.
I don’t believe we should travel to Karak Zorn. Anyone but Bardin wouldn’t have any reason to travel there whatsoever, and even he is hinted to not be searching for the Hold at all.
A Lizardmen Temple-City, likewise. No reason to travel there (beyond grabbing X artifact), and if we did, it would feel very empty without any Lizardmen actually there (and it would take far too much work for Fatshark to create several units of the Lizardmen that would only be used on 1-3 maps).
The Pactsworn threat is in the Reikland, so it makes sense that most of our escapades would take place in that immediate area. People like Kruber and Saltzpyre would probably be reluctant to travel far across the world, leaving the Empire defenseless in the mean time.
New heroes are the safest option, honestly, as long as the hero in question would fit in the base game of course. Bold of you to claim that only 3 options would make sense though, and then two of them is a Skink and Vampire, one of the most exotic and exclusive races in the world, and one of the most distrusted and viled respectively. There are far more options, and we shouldn’t try to just restrict it to a finite number (though, I won’t act like all options are valid. There are indeed those that are obviously out of the question).
On a last note, I wonder what you mean by “reinvention of the wheel”?