Considering the backlash FS got for Beastmen appearing on “old” maps because “it makes no sense”, it is hard to see simply adding a new enemy DLC. And I would assume that GW sees this similar. With that in mind, new enemies would most likely also need new maps alongside them. And to make this worthwhile we would be talking about something like about six new maps at the same time. Because creating a new enemy faction for just three maps feels wasted and probably not worth the effort from developers side. There might be an exception for the Chaos Wastes although they are primarly inhabitated by Beastmen and demons usually don’t appear in the region we are running around. At least not in large numbers so it would boil down to some like specific encounters.
As far as I am aware there are licensing issues with the engine which more or less prohibit the usage of a map editor by people outside of FS. They simply aren’t allowed to provide this tool (FS correct me on this if I remember wrong!).
And this is before we go into other issues with player created maps. Like they would have to be within lore and original content to be allowed to be played on official realm. And looking at maps from games like Left4Dead 2, it is also a fair assumption that 90 % of the maps released would be copy&paste from other games or just outright bad. And then there is also the issue with the engine not being well documented and not many assets being available. Most map makers, even those who make excellent work are just map makers and not creating new assets. Which would run the risk of maps feeling similar to what we already have. And we would have to use the same dialogues which would be only a lesser issue though. So, there are a lot of issues with that starting with licensing rights.
EDIT (for examples):
Beastmen Injected feels wrong (Anti-Beastmen Feedback) - Vermintide 2 / Vermintide 2 - Feedback - Fatshark Forums (fatsharkgames.com)
A suggestion make the beastmen fit better into VT2’s storyline - Vermintide 2 / Vermintide 2 - Feedback - Fatshark Forums (fatsharkgames.com)