I think Blizz do a pretty decent job in designing and keeping things balanced these days and I think the community does a pretty good job of figuring out how to use cards. Don't think there's any really "hidden" powerful cards that would pop out and change the meta if only given the chance.
If you look at it more nuanced and drop the requirement of them being more powerful than they should be, I think the Dragon and Pirate packages, with cards like Raid Boss Onyxia , basically the mini-set additions from last year, seem underplayed these days.
Also, especially these last couple of expansions there seems to have been a shift where now classes combine multiple packages into a deck. Warlock is running both Imps and Curses, Mage is running both Ping and Big Spell, Control Shaman ran both a Murloc package and a control package, etc. Multiple classes run hybrid packages like that. Doing that has meant that there's more room for players to just run all the good cards, instead of focusing on 1 particular thing and building with less good cards around that.
I think that is also contributing to less cards being left out, even if they are good cards.
You can look at Wild for those answers. The big 2 I can think of are Swordfish and Neptulon. Standard doesn't have the draw between Secret Passage and Cutting Class where a 12 damage weapon can effect the meta as much as it does in wild. Neptulon isn't seeing play in standard, but if something gets printed to rez or cheat it out, it'll get nerfed real quick. Cancer Priest can get Neptulon out turn 2 with coin, then 40 damage to face turn 3 with coin and the right draw.
I always thought that the 2 mana neutral gargoyle that buffs beasts was gonna see play in a top tier aggro hunter deck, along with the Hunter's location. I do wonder if there is a deck there the meta is sleeping on.
Also the 3 mana recruit card in neutral was and still is something I'm surprised hasn't taken off in aggro decks. Even getting a one drop, you're still playing essentially a 2 mana 2/3, a 1 mana minion and a draw for 3 mana. Absolute worst case scenario is still worth it. But apparently not..
Dinner performer from this expansion, it's technically not a recruit minion but it could've been. I'm on my phone and couldn't change pages without losing what I wrote so I couldn't check the name when writing my original comment 😅