Hey wait! I've got a new complaint!

Ok, maybe not that new.

Anyway, I was beating my head against a wall playing on the standard ladder earlier, which of course means mech mages and mech paladins til I vomit. In one game, I was doing fairly well clearing those damn bots from a mage, receiving only a few flesh wounds from the two mecha-sharks, keeping things from getting too ugly. And then. . . 3 turns in a row, Gaia, the Techtonic, one natural, two discovered. Any I thought of something that had been on my mind before - DISCOVER is very often a problem in games like this, and maybe that needs to be tuned up. I've run into 3 The Leviathans as well since the expansion started, and it occurs to me that maybe discover cards should be limited to their rarity gem, meaning if Discover in attached to a common card, it can only discover common cards. . . rare can discover up to rare cards, epic up to epic, and in order to Discover a Legendary, you'd need to add the keyword to a Legendary card, or make one. It wouldn't necessarily guarantee you that, but simply add them to the pool, maybe with a higher "find" rate. I just find it annoying that some dumb two cost Discover card is capable of, essentially, guaranteeing a win later on, without having to play well, or fulfill any other requirements.

Love it or hate it, I'm forever in debt to your priceless advice.

Its an unfortunate circumstance that will happen in every rotation because of the smaller pool available. This never used to be a problem because classic was sufficiently big and useless enough that most discover effects gets diluted fairly well. But core, featuring a smaller amount of cards, changed all that and we're at where we're at.

As bad as Amalgam of the Deep is abusing the smaller pool right now, at very least its a neutral card so you can easily abuse it back yourself. So while I think the card needs a stat nerf, its still kinda within what I could consider as fair.

Also, your suggestion doesn't really solve much since by diluting the pool according to rarity it can more easily be abused by something else like Venomous Scorpid or Pandaren Importer, and it makes the game more linear, which I dont think team5 would want.

Its much better to simply remove the possibilities of getting additional copies of certain cards. This is already in effect for quest cards. So why not legendaries as well?

The issue with this in mech decks is partly due to a small pool of mechs - as @dapperdog pointed out - but also a high number of cards that discover more mechs. All tribes get Amalgam of the Deep, but mechs have Gorillabot A-3 and Deeprun Engineer too.

Given my biggest gripe with both mech decks is their ability to discover more copies of things than you can hope to play around, I feel like adding the Gorillabot to the Core set was the real mistake. They knew mechs would have 4 discovers already, so why give them 2 more? They could easily have replaced Spider Tank with Harvest Golem, perhaps with a slight buff, and they would have given mechs the slight buff they were after without putting us so deep into this situation.

I don't think the rules of discover need to change though. The rarity aspect has been accepted with basically zero concern since LoE, making this a quantity over quality matter right now. Besides, tying discover rarities to the rarity of the card doing the discovering has 3 problems:

1. It reduces card pools further. Sure that probably makes it harder to find legendaries, but good luck with them finding even more Mecha-Sharks.
2. Discover has always been a mechanism for players with limited collections to experience using cards they don't own, especially epics and legendaries. Taking that away is actively hurting new and F2P players.
3. How would it handle instances where token cards do the discovering/generating? They have no rarity, and you cannot discover non-collectible cards.

Yeah, Discover will even out with the next expansions. It is frustrating now, but by the last expansion of the year the pool could get so diluted that discover becomes more of a crapshoot (or Blizzard can lose interest in mechs, not print any more of them, and we're in the same place).

Kibler had a video with a piranha swarmer deck in which he used the Amalgam of the Deep to always get his choice of anchored totem or flametongue totem; you could even hero power, swarmer, and then one of these. 

Just enjoy the ride. Any other route lies madness. 

Oh, and I AM forever in debt to your priceless advice.