I was thinking that it should be useful if any employee in a company, like the one that I work for, could log on to some page, then be able to manage a set of other employees (add, remove). The idea is that notifications about this set of employees will be sent (say, by email) whenever anyone from that set requests time off, changes time off requests, get approved for time off, etc.
Why is this useful? Because this way I can mark people who are critical for me for a period of time (users should also be able to define the time period too, not only the set of people that they are interested in), and then get notified about their availability. This is extremely important when working with several people and the schedule is tight and there are dependencies among people.
Strangely, I proposed this idea to people in IT, to my manager to HR people and everyone says "wow, nice idea, let's see who's going to do it" instead of actually doing it.
I think that this is a very important tool for employees to enhance performance by not losing time by waiting for peers that went on vacation without you knowing about it.