I am an advocate of reworking Thunderhorn. This card single-handedly invalidates Swarm, battle pets, and Lyonar’s in-faction ability of Zeal with its existence. It’s similar to old Blistering Skorn, which I don’t think was too strong as a 3-mana 3/4, but it was preventing other archetypes from being valid, just like Thunderhorn does today.
Is it ridiculously powerful? I don’t think so, but it makes games a lot more awkward when you have to play around this minion. Heck, against Vanar, you can’t have joined things on board at ANY point once your opponent reaches 6 mana unless you want to risk being punished. Playing around Thunderhorn is way different from playing around Blast or Frenzy.
I think this minion needs the Kron treatment. I like the idea of a 4/2 or maybe a 4/3 stat line. No Inner Focus synergy, and it can still capitalize on Wraithlings, as well as certain Battle Pets. But then it would need to make use of its ability to be good instead of just being a good stat stick at its worst. An ability like that should come at some risk to the user.