cursive bramble
It doesn't need to be fully endless maybe just 10-20 levels would do depending on many factors, just a place you can test your build once everything else its maxed out or maybe come a bit sooner for some early level rewards and leave it unfinished until latter with progressively stronger enemies each level. some big ass place completely optional to the main plot line, Pathfinder kingmaker had such DLC, Beneath the stolen lands and it gave me a really big second wind for that game.
Obviously you should be able to be a bit self sufficient on such place and not have the need to be constantly going back to your caravan in order to restock but that could be solved in so many different ways such as resting rooms between levels or plenty of randomized places with consumables or reagents to craft the stuff you need.
I encourage everyone to leave any ideas that can fit such a place for Stoneshard, i know DLC content its still far on the horizon but i see no harm in giving the devs some ideas already so they have a more focused direction once they are finished with the main release.
spiral matrix
fallow wren
dawn ledge