Absolutely agree with the comments about being able to revisit dungeons in a solo context. That's another way we could, at least in some way, get more bang for our buck out of the "only one dungeon" a patch arrangement. If they're only going to be spending the resources to make a single dungeon then it would be great if the dungeon we did get was not only better in typical dungeon form but was also utilized in extra ways. Let us go back in to explore for a quest. Let us see some of the areas that are blocked off during the multiplayer run. Heck, just let us go in and take screenshots or maybe use them for RP settings. Sure there would maybe be a bit more work in the environmental design aspect but there really wouldn't be any more work needed on the encounter design side of things.

If the waste in designing "extra" dungeons comes in a fear of them being "one and done" then don't just use them once.