I've only changed my avatar once, from a little pink cartoon fetus to the cartoon bat that I have now. So not many chances to establish any kind of theme, although both of my avatars did come from my own original art, so there are probably some stylistic similarities if nothing else (fetuses and bats are kind of different things). I just don't like changing my avatar a lot; I feel like my avatar is more or less my identity on any forum, so I'd rather just get one that I really like and keep it for as long as possible, while maybe switching to improved or otherwise tweaked versions of that same avatar from time to time.