Hearthpwn

Might be fun in a Highlander deck, a Yogg deck, a memey version of Questline Druid with multiple Guffs in deck post reward, or even Elwynn Boar.

It's decent, but not that good.  Since it's not select about what it discovers it's not as useful as Shadow Visions.  If you can't play the card it's very weak.

Very fitting for getting to Moonfang faster.

Interesting tutor for spells, and luckily it's not a nature spell, imagine this on anacondra

It's any card, not just spells.

Oh, you're right, so it can tutor key minions too for combo decks, even better

taking the original copy of the card you played in the same turn seems too situational, but maybe a combo player will find a way to abuse this effect

Better shadow visions?

Different. It's worse because you could build your deck to get a consistent card from shadow visions, and this finds any card. Better, because you can draw the card as well. I bet it will just inflate combo decks.