Deliverator
Graphical Hackificator
Why on earth doesn't that thing let you put your model together like a lego set, complete with the ability to import new parts, and then you just mash a single button to do all the ~stuff~ related to it? That would enable people like me (number changers) to be able to do something resembling basic modding.
The Asset Editor/Previewer already has elements of this. For example you can drag to position Tile Base elements (3D elements) in the Asset Previewer to make new Improvements etc of existing elements which is a really nice feature. For 3D units there should really be better tie in between the Asset Previewer and Units.artdef. You can put together you Unit from existing attachments (which is already possible) in the previewer and then press a button to create the Unit Member entry based on the Unit you are looking at - I feel like that wouldn't be too hard and would be a natural extension of what is already in place.
Also as the @thecrazyscot's tool shows the Mod.Art.xml could be automatically created/maintained based on the presence of XLPs and ArtDefs and the XLPs could be automatically created/maintained based on the presence and types of Assets and Textures. Art stuff could definitely be made simpler from a modders perspective for sure.
That said they have said they are going to continue to work on the tools and they have already put far more effort into this area than went into art support for either Civ 4 or Civ 5.