mine is that it has to be easily typeable for when I'm requesting dailies and badges, and if I'm honoring a bias, the aesthetic has to match that idol. Like, if my username sas still wonbunny302 (after Wonbin), I'd need to have a Wonbin layout to match. idk ig I'm just weird.
But another rule I have, especially after that stint with a certain short-lived username (not janniecomb, it was before that), it has to make sense both to me and to others. That's why I always ask people if I'm unsure of whether a username is comprehensible or not. (I'm not changing this one yet bc I'm trying to beat the time I had the last one, which was 36 days)