It would be great if we could attach prims to the vertexes of our avatars instead of the 'lines joining the vertexes'. Eg: attach a prim to your elbow instead of forearm. When attaching complex prims to your avatar it is almost impossible to make them fit well once your avatar is animated. An alternative could be to make it so that you can see the 'skeleton' of your avatar. (probably easier to program this)
Ps: I would love to be able to alter my avatar more, like removing or adding limbs (and thus making new animations) but I know this will take a lot of programming and is probably very hard to implement.