I've been mulling over about 4-5 ideas now for a couple months all the while looking at and scoring Threadless designs and submission. I think some of them could work - it's just a scary process.
Most people's criticisms over Threadless is that is incredibly hard to get your first submission approved and printed. "It is just a popularity contest" is the thing I keep hearing. Hopefully the work and idea counts for something too rather than who submitted it.
It is scary, but I do think success will come with enough work and persistence like most anything in life.