Not really an app but I'd like to see a system tweak which would allow the alert function to be more like an alarm clock function, louder and longer than just the little beep, with snooze function of various interval. The ability to change the alert on a per event basis would be fantastic as well. I'm sick of waking up on a vacation day bc I've forgotten to turn my alarm off, it would be soooo nice to manage all my alarm clock functions through my calendar.
I did see an app like this in the app store, problem being the app has to be running and multitasking will kill it when needed, and obviously those without multitasking ability are pretty much screwed.
If it makes any difference to any dev, I'd totally pay for this.