Personally I don’t see much point in adding yet another type of the keys. It most likely will mean we’d get less of the other keys “to balance things out”. I rather keep getting the normal event keys and gem keys, ascending higher rarity troops is difficult enough already.
If anything, I would suggest a re-work of the gold keys. The moment player has 4 copies of any given troop ascended to mythic (be it common or rare), reduce the drop rate of this troop from the gold key chest by 50% (or insert any other number you’d be comfortable with).
Removing them from the chest entirely would make it too easy to get to the point where we get pure resources (i.e. no cards at all) from the gold keys, so that’s probably not happening. But I think lowering the drop rate of the cards people don’t need anymore is a reasonable idea.
(I would also suggest lowering the drop rate of Major traitstones from the gold key chests (and any other source …) above lvl500, but that’s just my unrealistic dream of seeing less of those damned stones )