It's a bit of a no-no to have one player taking actions on their opponent's turn, which is perhaps why we've never seen Discover (an action that has to happen on the controlling player's turn) and Deathrattle (a mechanic that can be activated on either player's turn) combined together. It seems to be a bit of a cardinal sin in fan-creations as well to do deathrattle-discover, because it's murky how it would work— am I supposed to be doing something on my opponent's turn and taking up their time? It muddles the whole concept of 'turn'.
But no longer!... kind of. With KFT, cards like Skelemancer and Vryghoul have introduced the concept of 'on your opponent's turn' deathrattles. With this, you can make cards that have Deathrattles that only do something if you make your opponent kill them on their turn (and terms). If we extend this concept, we can also make Deathrattles that only activate if it's your turn.
But what about Discover? Well, sadly I don't think there's any way for one player to discover on the other player's turn and have it be fair or fun. Instead;
Bone Wyvern acts as a kind of a counterpart to Bone Drake— even with a higher base 'power level' if you like, this card might actually be worse than it. It has to die on your opponent's turn to do anything, and your opponent gets to choose what it gives you— likely giving you the worst of the three options.
I did mention that Discover Deathrattles could happen on your turn as well— so how about it?
With Shadowfallen, you get a taste of how deathrattle discover could help Quest priest. Warlock cards could be pretty good in Priest given its life-regain capabilities, and Kabal Courier gives you a clue as to the balancing. The design philosophy I have for Deathrattle "on your turn" vs. "on your opponent's turn" is that the Attack and Health should be reversed— more Health for your turn, less Health for your opponent's, as it increases the chance of activation on your opponent's turn, but makes it harder to activate on your own turn.
But the wording? Surely Bone Wyvern is too wordy?
You might be right. Dread Inquisitor uses 'for you' instead of 'to put in your hand'. Fewer words, same meaning, right? There are a few bad-egg Death Knight cards in the first place, so I think this card is balanced when they give you Obliterate when they have a board full of high-health minions.
Here's just one more example:
With Fossilized Bear, you get higher health with higher Cost to justify the effect. Works more like Stitched Tracker than anything else— you discover the card and you get two copies— you don't discover twice. But who said it had to be just Discover? There are a few Discover-like effects that could work in this way:
These two cards operate along similar lines to Discover: Zombified Taxidermist has a similar interface with Build-A-Beast, as does Lightstruck Skeleton with Adapt (three choices out of 10)— for that card I think they get to pick which of your minions to adapt.
The third, Imp Hoarder, does something different— it collapses the 'if it's your opponent's turn' and the 'Inspire' mechanic into one thing: Enemy Inspire: "When your opponent uses their hero power", which is by default on their turn. This removes the necessity of 'Deathrattle' being the operative mechanic that this could work on. With the fourth card, Risen Blade, we have an instance of "if it's your opponent's turn" folded grammatically into the Gahz'rilla mechanic. Admittedly, as you go further away from deathrattle, it gets harder.
Q: What happens if your opponent has Barron Geddon on the board and I have a 7/2 Bone Wyvern? After they end their turn, Geddon's effect will go off and the Wyvern will die... will they have to discover a dragon for me?
A: Since your opponent's turn is over, they don't get to discover anything for you, but it's not even your turn yet— merely the end of theirs. What happens in this instance is the deathrattle doesn't go off and nothing happens. To be honest though, I'm not even sure what happens when you have a Skelemancer and your opponent has a Barron Geddon– presumably it summons the 8/8.
Q: What happens if I have a Doomsayer and a Shadowfallen on board, and my turn starts?
A: Your Doomsayer activates, your Shadowfallen dies, and you discover a Warlock card. This is because 'end of turn' effects activate once you press the 'end turn' button and give up your ability to interact, and 'start of turn' effects only occur once that handover in interactivity is complete.
All in all, I think this is a potential implementation for the Discover mechanic (and others) when paired with Deathrattle (or even other mechanics!). Give me your thoughts, and post your own 'deathrattle discover' cards if you're struck by creativity.
Couldn't Blizzard announce deathrattle-discover effects in a new expansion and then explain that at the start of your next turn, you can pick? There can be a little indicator somewhere in small but readable white text that says what card the deathrattle-discover is currently being looked at by the player. It doesn't seem too hard to implement overall.
And then, your ideas can also be implemented in a new expansion after that one once Blizzard is confident that the player-base understands how deathrattle-discover works (because they have to assume that everyone has a 1-2 year learning curve on new effects lol).