I very much doubt we'll see 3rd party UI mods. They are very strict over what they allow (one of the reasons that the UI was so slow at launch), so I really don't see them letting people have free reign over the UI. They are against anything that would trivialize content, and any mod involving meters or calculations do just that. So if we see any modding available at all, I can almost guarantee it will be only in the form of skinning and appearance mods.