I have one for bosses in large group content (8m and 24m) that changes the boss' target marker to show the countdown. Gotten a couple compliments on it, but it's useless in any situation where you might need to mark adds.

15s is a long wait. Unless you're doing hunts, people probably aren't going to wait that long for you to pull. The one I mentioned is only 5s long, and even then, people tend to cut it off (because DPS like to feel important by being the first one to hit the boss or something).