Guildpact, Rare. Hatching Plans seems like a pretty straight-forward card. When it hits your graveyard you get to draw 3 cards. And drawing 3 cards for the cost of one is a big win for any deck. What makes Hatching Plans really interesting however, is how you kill it off!
My personal favorite card to combo with it is Phantatog. You sacrifice Hatching Plans to the Atog, he gets +1/+1, and then you get a new grip of cards that you could decide to pitch to pump him up further. Even if you don’t discard those new cards, chances are you now have some new shiny thing with which to ruin your opponent’s day.
If you want more of a control or stall strategy, you could go for Faith Healer as your Hatching Plans enabler. Gaining 3 life and drawing 3 cards just by sacrificing Hatching Plans sounds like a pretty dang good deal to me, and if you’re running Blue/White, there is a whole slew of things you do with an enchantment theme. Heck, you could actually combine both options I’ve mentioned here!