Alex: "It's very cool to finally see that ability on a printed card, but it seems somewhat drab for a mythic having seen the effect so many times"

If no one has any creature cards to reveal, ": you win the game" is just as common as trample, haste, and anything else. Then again, so is "You can't activate abilities of creatures" and "If a player would gain priority, instead that player takes a shot."

Beyond that highly theoretical exercise, I don't see any reason why someone using this card would want anything other than a massive tie at 1. Memory issues are an enormous problem here too, since the cards go back to hidden once the trigger resolves, and neither player is thereafter entitled to check the other player's cards.

That was my thought too. Perhaps it needs to have something like "......."

I just realised how hard that is to write and make sound intelligent.

Well, I was going to make it "an ability of your choice from among the creatures revealed that way" which does avoid those problems. But that's less silly and mythic and answering the point.

See, now, if we had 'like' buttons, I would just go around abusing them all the time.

Was: "Marshmallow Griffin has whichever abilities are most common among the revealed creatures."

Wow, wordy now.