I would want it to be super cute, but durable so my knitting needles don't poke through.
I would want it to have little details and touches that make it feel like it was made just for me.
I would want a my sock project to fit inside with enough space that I don't have to worry about snagging the yarn and definitely enough space for two at a time socks.
And, of course, the inside would have to be as funky and fun as the outside.
Literally, the first thing I thought after opening this bag and putting my latest project inside was that I needed another one. (Maybe Dr. Who or Firefly?) I have another sock project that I keep at work that defintely needs one of these.