From 3d printed clothing, the obvious next step should be to have your phone take a 3d scan of you, and send it to the clothing designer to print it to your actual body size and shape. We could have truly unique sizing (none of this S/M/L/XL stuff)!

Edge case: people who are in the process of changing their body size/shape. Growing children, people losing weight, people gaining weight (they're out there), will all occasionally want to buy for where their body is going to be in the future, not where it is now. How to accommodate them?

Yes, and people have been chasing that Grail for decades. It's always right around the corner. (Despite what another poster said, it IS being pursued commercially. And unobtainable so far.)