Either up the cost of the spell cast in return, e.g.
<pre>(...) cast a random spell that costs 1 mana more.</pre>
Or restrict the trigger for the secret, e.g.
Whenever your opponent plays a spell that costs 3 or less (...) Whenever your opponent plays a spell that costs 3 or more (...)
The first one in theory can make playing a spell into it still worth it, though the balance is difficult here. At 1 mana more Yogg still feels powerful, the resulting spell still most likely will be garbage, inconsequential to the game state. At 2 mana more Yogg becomes a questionable inclusion in a deck, because it can present a serious tempo swing for the opponent or an opportunity to gain a lot of value.
The second one I feel is the better approach. Yogg no longer counters the whole spectrum of spells and while it still is powerful, there are spells that escape its effect, therefore it becomes less valuable in some situations and against some decks.
I wouldn't nerf Oh My Yogg! ... just like I wouldn't nerf Counterspell (arguably worse to have to play into because Oh My Yogg can provide benefit) ... I once played into what I thought/hoped was Oh My Yogg with a 5 mana C'Thun spell ... I was hoping for the 5 mana buff your board and summon 2 treants ... and voila, game me lethal where I had no other options for lethal. :)
5 Mana spells are very limited at the moment, most are fairly decent.
Also, it is pretty easy to play around ... not like the Mage secret combo of Counterspell and Netherwind Portal, where no matter what you do, it is of benefit to the Mage.
What I would like isn't exactly a nerf but it would act as one for oh my yogg and any future card with similar design.
For all disruption effects that turn cards into other cards, or maybe for all the RNG, I would like cards to be separated in different buckets. For example, all buff cards together, all damage cards together, all deathrattle together, all spell schools, all minion tags, etc.
This way skill would actually matter when playing around these cards, there would be a real learning curve against RNG effects and any human could reasonably try to guess what will be the outcome based on which card they use, instead of not giving a f*ck because nobody can play around 500 cards for every RNG effect.
Similarly, if you Devolving Missiles a beast deathrattle minion, it would turn into beast or deathrattle minions that cost 1 less, but never into a battlecry or a mech. Bad example I guess because it could turn into a mech deathrattle or a beast battlecry, anyway you get my point, buckets.
That would require to rework a lot of the game code I imagine, but that would make RNG so much more interesting and balanced imo, and even give design space to the devs. I'm sure this would also make some cards stronger due to an over-restricted card pool, kinda like Primordial Studies currently in standard mage decks, but I don't mind.
I would also like if Oh My Yogg! kept the original target when possible.
My thought would be, "when an opponent plays a card they instead play a card of the same cost"
Still feels powerful and flavorful, but easier for opponents to combat. Playing a random minion is much less downside than a random spell.
I don't think a nerf is possible. I think the design of the card is so narrow that it probably can't be redesigned to be something similar but weaker.
Making it "plays any card" means that it counters minions, too, and that'd potentially be just as strong. The more I think about it, the more I really hate that idea. Imagine a Paladin having a board lead going into a later-game turn, and dropping OMY. You can't play your removal spell, since it'll become something useless, but you also can't play a taunt minion, or a battlecry. Unless able to waste something cheap and still afford what you want, there's literally zero that you can do about it. If the only thing you have is an on-curve play, you're just entirely ****ed beyond belief. The status quo counter-play option for OMY that you can play minions without interference is entirely gone, and that'd be a problem.
Making OMY cast a spell of higher mana cost also still doesn't work. Still often is 1-mana Counterspell, but the worst-case highrolls just got a lot worse.
If OMY is enough of a problem to do something about, I think the best thing is simply to axe it. Gone. Other games have had bans on cards that just don't work out. Bring back spells giving your minions +2 health or Eye for an Eye or something, if Paladin needed another secret after removing OMY. Could that seem drastic? Maybe, but being willing to delete minor but incredibly annoying things is often more important than removing the most powerful cards.
I don't think it's that unfair compared to cards in other classes. Mage can reduce the cost of all cards in their entire deck and then draw cards while replenishing their mana. Rogue can spend one mana to return a minion to its opponent's hand and make it cost one more and rogue can also easily draw through their entire deck.
Sure, counter spell costs 3 mana in theory but mages often find a way to get it out for 0, 1, or 2 mana.
I don't think it deserves a nerf: Is it a good card that can be frustrating? Yes. Is it an OP card? No. Is it a card that allows a skill-dense play-around? Like few others. (at least for tactics)
It's a frustrating card because sometimes you can't really play around it and because it has a wide range of possible outcomes. You also often lose the game on the spot. But it is a card that rewards the skills of the player and this is an important factor for the game. And this also means that the frustration level can be largely reduced.
This card involves: - a knowledge of the card pool (especially now that it is at a minimum) to look for unthinkable outs - the ability to "recycle" a spell not useful in the MU with a random spell of the same cost - the ability to play around it on multiple turns for a game plan (this card particularly punishes when you don't have low cost spells and you need to play a spell that is "too" expensive, so often when you're behind the board and you cast an aoe, or a spell to contest board; you can play game plans that avoid these scenarios).
With this information you can play around it - even in deckbuilding - reducing frustration and increasing winrate. You'll still lose to them sometimes, but maybe you'll gnaw less. Think about next game and have fun.
I wouldn't. This crying for nerfs all the time lately feels like a cousin of cancel culture.. blizzard already did their usual roundup of nerfs after an expansion dropped. Adapt and move on. Also, no I do not play pally.... I just see it for what it is