You said basically what I was thinking. The rendering abilities of the current WoW art team are incredible when you go to newer zones and newer content. The older zones really show their age sometimes in comparison. The tree trunks are like card board paper tubes and the textures are just wonky. Just redoing what is already there with the new art teams ability would be a major upgrade.
Heck even making more buildings that you can go inside in old zones would be nice, instead of just a shut door. Like how BFA had a lot of nice cozy homes you could explore that were fully furnished with knickknacks, paintings, furniture, and even shelves on the walls! Could place knickknacks themed to the zones they are in or the story lines in the area. There are a lot of hidden away little areas in these old zones like forgotten camp sites, hollowed out trees, and hidden mystery spots that would look amazing if the new art team could just fix them up structure wise and give them better coloring.