Not completely opposed to the idea of being able to further upgrade troops, unless it needs to be done by fishing out more troops randomly and ascending in the traditional manner. Plus, even the notion of maybe having a future base rarity even rarer than base mythics are is not something I ever want to deal with, at least not with the drop system the way it is now.
If a new rarity tier or just "shinyness" were added that did contribute to stats, I hope the upgrade cost would be some combination of battles fought, souls, traitstones, and gems. If the gem and/or battles costs were reasonable, I'd totally go for as many as I could (souls and traitstones can be borderline unreasonable as far as I'm concerned at this point, so long as I get to use them, since I can always get more and it would give me something else to earn over time).
Cosmetic only, it could be for just using the troop.. or gems. They have to monetize it somehow, after all. I probably wouldn't go out of my way to collect these, especially if they had ridiculous battle count requirements, but they'd be nice to see on troops I did actually frequently use.