Yowch. Seems a bit easy to game, with anything from Spin into Myth to Cerebral Eruption to any clash card. Given that you're getting a Beacon of Immortality plus a universal Vorel of the Hull Clade activation, that seems a bit too cheap.

Mmm, maybe shuffle their deck first?

Though this would be kinda hilarious against infect. (Double my life and, uh, ooops.)

That'd make sense. A shuffle as the first step of the resolution would be a good tweak.

At that point you could even make it apply to your own library if you wanted. That'd be easier to game (using things like Shadowborn Apostle or Selective Memory), but that might be what you want. It's obviously fine to leave it at shuffling-then-guessing the opponent's top card too.

Eh, it was designed for laughs against Vampiric Tutor and other cards. Shuffling makes that kind of useless. It was oversight on my part not to think of cards like Alex mentioned.

In which case I suggest you take the reward down a bit. Predict and Mise already play in this space and work okay. I believe Booby Trap and Letter Bomb were sortof intended to do a similar thing too.

I can't help but noting the percentage of un-cards that Alex is using to justify this :)

But yeah, doubling is kinda huge. Even just guessing "Island" against a blue deck, for that 1-in-3 chance of, what, 12-20 life?