Having put the New Xbox Experience up, Microsoft has promised that it will soon increase the “fashion selection” choices for the new customizable avatars.
"Come back every two weeks to check out new items. We’ll be doubling our fashion selection over the next few months," reads an advertisement found by Xbox 360 Fanboy. It is not known if the new content will be free or not, but Microsoft has confirmed that they will come out in “theme packs”.
Microsoft is also currently planning unlockable avatar items based on in-game achievements. More on this as it develops.