Would be cool to be able to leave a "stamp" -- some sort of minor illustration -- on the scenes for any eyes to see. Perhaps these would be little colored butterflies, flowers, mushrooms, skulls, footprints, fingerprints, cigarette butts, bloodstains, torn hem of cloth, etc.
Would be massively useful in leaving trails for clues for mystery quests.
They would obviously have to expire in a specified time-frame.
There is probably going to be concern about screening the allowed stamps - so I guess there would have to be an approved list or pallet that grows over time.