Many users do not like some types of shapes. Possibility to turn on rectangle avatars is a good idea.
It can be something non-standard like here:http://www.getsymphony.com/discuss/thread/106489/1/
Probably won't happen . If people wants to change the page layout/design, they can do it themselves by creating a new theme or modifying the default one.
Stuff like changing button colors, etc is something I don't want in the admin cp. A place to put custom CSS code would be a better idea, then you can change the avatar shape yourself, if you know CSS.