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There are three groups of unlockable stage parts.  These are easy to unlock.  Play on a created stage 10 times to unlock the first group of parts.  Build 5 stages to unlock the second group of parts and build 15 stages to unlock the final group of parts.  These new parts should make the stage builder mode a lot more interesting.

I have to say I was overjoyed when I found out that this game will have a stage builder. One of my favorite features in any given video game is to create my own levels. There's nothing quite like running around on a landscape wrought by your own hand. Thanks for the info. Those extra parts will really come in handy.