The devs have said their intentions were to have all of the class art work used as game loading screens, but it was more of a resource issue.
Given mamba’s post noting the higher quality image of the Necromancer wall paper than the other masteries, lends me to believe that it will be a new loading screen.
So easy choice! BOTH! LOL!
Edit: One hopes that all of the mastery wall papers will be redone to be used as loading screens in the expansion, or just as an update.
Edit 2: Just re-read the OP and I’d like both, not just one.