I would like to see an option in the UCP under the profile tab where you can upload your own cover image. So when you are viewing a user's profile they can have a custom cover photo.
The position I had in mind is at the top just below the "Viewing profile - Username" text. Similiar to what you would see on facebook, twitter and more. The size should have the same width as the form#viewprofile.
I am currently using phpbb 3.0.12.
(If there already is an mod for this, please let me know).