I'd love to have an app in which I could set multiple timers to run concurrently. This would be most useful in the kitchen (40 minutes for the chicken, 10 for the noodles, 3 for the tea, etc.) but I used to use the Pebble version daily to keep myself on task (1 hour to work out total, 20 minutes to cycle, 15 for weights, etc.).
I don't know if Garmin lets you run apps in the background, but it would be nice if you could check your notifications and such while the timers continued to run. This ability really helped after I had surgery and needed to have multiple medications on individual 3-4 hour timers. I was able to keep using my Pebble like normal but rely on the timers to remind me when it was time for more drugs.
Anyway, thank you to the app developers in general! Garmin built a nice watch, but its the folks who build and share apps that have really made it useful beyond just tracking workouts.