A way to hear invisible characters.
From what I've seen in game, when you roll a listen check against one's move silently check, they're supposed to appear as half-transparent. Technically you aren't supposed to know who the person is just by hearing him/her, but that part's up to the way one role-plays. However, when someone's under the veil of invisibility, even if they came next to you, you can't keep track of their location, compared to an ooze who can follow your track if you come by too close or a monster which hears you when you do an action on an object, i.e. picking up plants.
Making a way for PCs to hear invisible characters would also be a nice counter for those without any casting ability, or even save a spell for bards who are proficient in listen skill.
It's been on my mind for some time, so I'm just throwing it out there.