"It's strong, but I personally don't think it's unbalanced. It means players have another option for reset, and what's good about this is that it's adapted to be useful for different situations from the other resets. You want VM if your deck is slow and you know you'll need a complete wipe at some point to reset the game. (With that said about half the time I put Valar in my deck it ends up biting me in the ass.) Wildfire is good if you feel your bombs will be able to outpace or at least match your opponent's bombs. It also works well with control - Bara kneel, Stark's Marriage Pact, etc. - which can neutralise big characters. VD simply means you'll need to curve out your deck a little bit more - make sure you can have your 7 bomb out with a 1- and a 2-coster so you still have solid board presence and some military protection. And your characters aren't being killed or discarded, so with a shuffle effect there's a reasonable chance of getting them back.
What I like about this plot is that, like Duel, it prevents the opponent with better economy from barfing out bombs with impunity. And while it looks like it would be good for weenie decks, and it is, there aren't many decks out there I'm aware of that rely solely on 1, 2, or 3 costers. Even NW KoS needs 4 costers to really get rolling."
"there aren't many decks out there I'm aware of that rely solely on 1, 2, or 3 costers"
I understand your doubts, but its not about having no big characters at all, it is about how much you rely on them. If you have to return a 4/5 coster in the deck as NW, you probably wont be too sad about it, but if I have to lose Cersei or Tywin with power tokens on them, or Balon/Euron/Victarion, I received such a heavy blow to my win condition. If i have to give up Deanerys but get to keep my dragons and burn, while my opponent loses key characters and crucial power tempo, I will always take it.
So the main problem is, houses like stark lannister greyjoy, and to some degree bara/tyrel, do not really have lower cost characters that can carry the game. Their lower cost characters are for the most part designed to support the big ones, and they have great support characters, but Caitlyn (Core), arya, bran, etc. cannot carry the game, so for example Starks literately must include renown high costers to win the game. At the moment there is no other option. And when you do it, when you play few high costers with dupes and bodyguards (because you dont want them lost to valar/varys).... BAM this plot goes off and you lost the game because you lost too much tempo,power and cards with dupes/attachments.
That is where my concern for this plot is at. On the surface it might looks like anti-First Snow that works on big dude decks, but its much stronger than that because it can not be saved and those characters don't return to your hand, and because those high costers will often lose some power with them that is also part of your decks win condition lost. As a consequence, these factions might perish from meta for some time like Tyrel, NW and some other decks did when the First snow was released, and of course if my maybe-rather-extreme opinion turned out to be true, it would be bad for our diversity of our games.