Often I find myself wondering how a particular person ended up on my friend's list. Being able to tag people on my friend's list with custom text (perhaps when hovering over their names) would be a very useful feature. You could tag "Raid" onto people who frequently raid with you. You could tag "PvP/Skirmish" with people who enjoy Skirmish more than anything. You could tag "Loud" to people who don't know how to adjust their mic volume. I think this would be a relatively simple add (as simple as patches to Destiny can be, anyway) with a pretty decent use. RES (Reddit Enhancement Suite) has a similar feature, for those who are interested in the concept.