I've got a few plans and visions, but my only true design decision is to keep it fun and accessible. I make sure to play the game a lot, and I've blocked my share of tigers. If something ever doesn't feel fun, I'll remove it. I believe that I can combine enough fun, accessibility and blocking for this game to be a huge melting pot of emergent gameplay.
I strongly believe that all good stories have a conflict, and that all good games tell a good story regardless of if it's pre-written or emergent. Free blocking mode is fine and dandy, but for many people it will ultimately become boring once you've got it figured out. It's like playing a first person shooter in god mode, or giving yourself infinite funds in a strategy game.. a lack of challenge kills the fun.
For survival mode, I'd rather make the game too difficult than too easy. That also means I'm going to have to include some way of winning the game (or some other climax) to prevent it becoming too exhausting.
But if it's no fun, I'll redesign.