Anything without the xX, 29343434, or -~ kinds of things. Usernames like Chibi [whatever] or Dark [whatever] are also not very memorable or desirable to me.
That being said, even though I have a short username without any of that, I'm not really into mine. One weird aside is that, even though when I joined there were no other Kaeths on Gaia (the name is made up, obviously), it somehow became a thing afterward and now there are 50+ usernames that contain the "Kaeth". At one point there was this group of friends that had coordinated their usernames around the concept of "Kaeth" ("Kaeth of Death", "Kaeth of Light", "Kaeth of Wind", etc.). They contacted me out of the blue as the "original" Kaeth and we would sometimes chat. It was kinda funny but also confusing because like, the name doesn't mean anything.