I have always loved the idea behind this card, and was immesurably disappointed with its execution. It's just so weak; the comparison that comes to mind is Renounce Darkness, which was such a fun card to play -- not too strong, but kind of okay as far as power goes. I like Explore Un'Goro more because I love the discover effect, as it is somewhat controlled randomness, and most of the cards with discover are not really broken either, while still being relevant. Just think of Venomous Scorpid - great card, relevant neutral minion, but doesn't break the game or even comes close to it.
In my opinion, Explore Un'Goro should have had a really unique effect, and perhaps they could have given it away as the promo expansion card, similar to C'Thun, Silas, Galakrond(s) etc. Thematically it just fits so well, too, like a call to action. Plus, I do believe that these promo cards should represent the set as a whole, have a fun effect, be playable, but not be broken (so no Priest Galakrond, for one).
As for the actual card, here is how I imagine it. It's a legendary spell, most likely neutral (I know, neutral spell, bear with me), and works the same way Whizbang and Zayle do, i.e. you put it in your deck, and that's the only card you have there. When the match starts, you have no deck, but instead at the beginning of your turn you discover a card from the Un'Goro expansion (Explore Un'Goro, duh), from any class. Your starting hand could consist of the Choose Your Path cards that the existing version gives you, but I would probably make them cost 0 still. (No danger of 0-mana spells in combos, as you don't build your deck.)
I think this would make a fun and balanced (certainly not too strong, but at least perhaps playable) card, and it fits the theme quite well, too: you abandon the battle to explore, well, Un'Goro, and you use the stuff you come across to fend off your pesky opponent. You would probably lose most of your games, as this is a bit like playing Arena on ladder, but still, at least it's fun. Any thoughts?