(1) Some ideas:
(a) make great examples of stuff using Expo that people like and make them open source so people can learn how to make them.
(b) Make libraries that are compatible with expo that people can npm install, like react-native-deck-swiper
(c) make videos showing how to build stuff with React Native and Expo or give talks about it at meetups, etc.
(d) Submit PRs to add stuff or fix stuff
(2) A way to have a debugger running in the JS VM on the actual device and a way to connect to that from a remote server, so you could write a great debugging tool that you could have in a web browser in your computer.
(3) We are looking at it. Aside from using Dart instead of JS and a home rolled React-like component system written in Dart instead of React, the really important difference between Flutter and RN is that Flutter renders straight to GL rather than trying to leverage native OS level components or a webview or anything. There are some really nice properties to this and it might be a better approach overall, but it requires incredible fidelity to match the user's expectations. Google might be able to pull it off but even if they do, keeping up with the platforms will be tricky. I think there is about a ~30% chance that that approach turns out to be better. We'll see.
(4) Someone might be! Brent cares a lot about Reason!