I have a recomendation for Halo Waypoint in the future. I don´t know if it has already been requested.
My recomendation is that we can be able to customize our profile photos at Halo Waypoint, import images from our pc or stuff like that. And that we only have one chance forever in order to change the profile photo to an imported image, so we have to select wisely.
It would be interesting, but I feel also bad because the moderators would have to see all the profile photos and see if they are ok (no -Yoink!- or things like that), and add a bit of freedom to the user.
Your last paragraph is probably the biggest reason as to why this hasn’t happened 
Sounds good in theory, but the work involved in making sure things are kept clean overwhelms the benefits it would bring. I kinda like avatars just being Halo related, maybe in future if we had a bigger pool to choose from - broken up into sets or something.