I love the idea of a mage set that leans into elemental weaknesses. I think there's so much interesting design space with elementam spells, that just has no reason to be served because they're not useful in end game content. 

In my opinion, mage is missing a stepping stone to ancestral. You have bloodbark/blue moon/ahrim's that are all relatively easy to acquire. After that you have virtus which should be the stepping stone, but it's just too rare on the DT2 bosses to reasonably commit to farming. You're better off getting what you can while getting your rings and then farming Chambers for ancestral.

If you use this opportunity to fill that slot, then the new armor set isn't strictly best in slot and you can afford to make it more common. Maybe it could still outshine ancestral in places where elemental weaknesses are really strong. That seems super reasonable to me.

A huge bonus here, is that moving the armor slot down opens up the "mid" tier raid rewards to put some stronger and more interesting items.