Something I have thought about lately is internationalization. I'm just barely getting started trying to write an app and I realized that the vivoactive (and other devices) are international devices with users from all over the world, however, I know one language, US English. I could easily make my app know the difference between miles and kilometers, but I wouldn't know the first thing about supporting Japanese, German, etc. Otherwise I would have to rely on Google Translate or other services that can directly translate words but generally miss things like grammar.
My suggestion for Garmin would be to add a service (hopefully free since so far I have yet to encounter a paid watch app) so developers could submit a language package in their native application for their watch apps, faces and widgets, and Garmin would supply language packs for other languages. Watch apps are simple things and I'm guessing that even the most complex apps would only have a couple dozen strings and the majority would have fewer than that.
I think this would give the entire platform a boost, allowing you to reach many other countries.