Believe it or not, this came to me in a dream…
New ascension level of Gold Mythic, costing double the # of troops for Mythic ascension. E.g… a base common Mythic troop would have to sacrifice 200 to ascend to base common Gold Mythic. Such ascension is reflected by having a gold border on the troop card.
No skill increase for Gold Mythic ascension. Instead, all Gold Mythic troops gain an identical Gold Mythic trait: upon death, a Gold Mythic troop will regenerate as a copy of a surviving random enemy troop. This would mean a Gold Mythic Fortress Gate could regenerate as a Maw, or vice versa.
And yes, Virginia, this feature would make speedruns in the meta a LOT harder, which is kind of the whole idea.
EDIT: In case it wasn’t obvious, a regenerated troop would not be Gold anymore, otherwise matches might never end.