The next Achievement total they pass will unlock a special box for the game, with a hinged lid and magnetized to stay closed. The ultimate goal is a secret.
From the campaign:
We’re halfway done and we love you with the entirety of our gutmeats, so it’s time to talk about STRETCH GOALS. Stretch goals can cause teams to lose focus, delay shipping, or destroy projects entirely. So we thought we wouldn’t have any, but you really seem to want them, and we love you from our sweat glands, so... starting today, you’ll see this on our Kickstarter page:
Unlocking achievements will earn these stretch goals:
Happy unlocking, backers.
We love you with every morsel of our blackened, charred little hearts.
-The Exploding Kittens Team