I think there are enough dungeons, they do really well at putting out that kind of content, above anything else. The problem is you don't get enough mileage out of them, you don't get enough hours which are fun rather than forced grind, because they are exactly the same every time you play, with the same enemies organised into the same group formations.
With this approach, the player quickly becomes fatigued with even the latest dungeons that were added. The game wants you to play them many hundreds of times over to achieve certain goals such as fully gearing your character and upgrading relic weapons. Even if you take your time in doing this casually, it still amounts to far too many identical dungeon runs, a routine like being a factory worker.
At the very least, I would like to see dungeons designed with a few different possibilities when it comes to enemy numbers, locations and formations, so that you don't know for sure what is coming around the next corner, it could be a pack of dogs, it could be the skeletons this time, nobody knows. This element of randomness will encourage thinking instead of following the same routine over and over.
I think this kind of change is very important to maintain player engagement especially when instanced dungeons and raids are the main focus of the game. Mixing up enemy numbers and positions is a simple change to make, while the current offering of just one version of each dungeon is quite wasteful when they could get more mileage out of these nice locations they've designed.
For additional excitement they could add some wildcards such as larger enemies that do not appear every time and offer additional rewards for slaying them, or rare alternate color versions of enemies that usually appear which have altered behavior in some way.
In summary I would like to see simple changes which make dungeons more dynamic and fun, to make players enjoy replaying them many times. I would prioritise this over adding more dungeons, for I feel it is not sustainable to keep adding locations in which the player has seen everything that can ever occur there in just one playthrough - that kind of design does not stay interesting enough to keep people engaged from patch to patch.