>Still no built-in way to collaborate with other musicians, it seems..

Asynchronous collaboration, there are a few non-native solutions.

Or you can always exchange files via Dropbox or something as you said. Or share screen.

Live playing over the public internet, there's no good way to do it , and can't really be due to latency. Many projects tried, all suck.

Live playing locally, there's Ableton Link.

This is a huge update (ARA alone, the step sequencer, etc) anyway, and many long awaited and several non expected items.