Sync: blocking, async non-blocking. I'm interested in this for a specific API design. Communicating with other Android apps is necessarily async. But I'll think about it.

I see the advantage of using a device relay but I'm unsure of the UX. Are you going to ask your users to download and continually run citrine in order to use your app? It will also depend on the overlap - for Amethyst and Zapstore it's probably none