Ability for every user to set custom display names of *other* users.

There is issue for making UI/UX more messendger-friendly (#1646). But I also do not see now possibility to set my custom display names for other people I'm chatting with, as I can do with other instant messengers. I have found in issues for Vector/Matrix that it will be good to have address book/contact list/roster, but nothing mentioned that user should be able to see other users with custom names and not the display names which that users choose for themselves.