I'm wondering about this, too. I wanted to change my username when I started actively posting (I was exclusively a lurker for years - too nervous to post anything). I just used my two main obsessions at the time I created my account, but I've thought of something better. I tried adding a nickname and selecting it to be displayed, but nothing happened.
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Yet in my new wildness and freedom I almost welcome the bitterness of alienage. For although nepenthe has calmed me, I know always that I am an outsider; a stranger in this century and among those who are still men.
-H. P. Lovecraft, "The Outsider"