I like the idea of a selective discard spell that replaces itself if it misses.

I like it too. But this seems rather cheap when compared to Ostracize. Admittedly, Despise also exists. But the big drawback with both those spells is the possible misfire. The addition of being able to discard a Planeswalker, while nice, doesn't make the card 'always useful, play four'. Eliminating the drawback, but not adjusting the cost seems too good to me.

If this was asking for something more specific, though, I could get behind it at a cost of . A creature with a converted mana cost of 3 or less, for example. Keeping in mind that it's still going to draw a card when it fails, it's still a great tool... it's just best when hunting a certain kind of deck.