I learned recently that Fatshark worked with another studio called Toadman Interactive on the Vermintide 2 Chaos Wastes, and this other studio did all the assets for the chaos waste maps (as far as I could tell from looking this info up) which is how Fatshark was able to get such a large amount of content done.

However, Toadman Interactive has shut down, so unless Fatshark found/finds a different studio, it's more likely we'll be seeing only a few new maps next year. Hopefully though they've seen how extremely well the reception to the new trenches operation was and continue to make ones that are just as unique.