You've given your reasons, here and elsewhere, why you wouldn't consider another expansion with potion-cost cards. Is there a chance we'd ever get a potion-cost promo card?
I'm not DXV, but the issue there is that it'd be a promo card dependent on another expansion, which means it would be useless to a lot of people. Not to mention many people disliked Potion cost cards, for one reason or another. Or in other words, as a promo it has mostly the same issues it would have in an expansion.
You could have it interact with Potions in a way that doesn't depend on them, maybe (similar to Apprentice), but that's about it.