> A question I didn't get to ask while there is -- will there be some sort of type sharing system introduced?

Yes. This was one of goals of our talks. We wanted implementors of type systems for ruby to start collaborating, in particular on a repository of typed shims.

The details of how this would work out might be different. Typescript and Flow change syntax and thus they cannot be included inline in arbitrary JavaScript libraries. We're intentionally compatible with Ruby syntax. Thus we would like this "repository of types for libraries" to only contain types intermittently, until types have been accepted to upstream of respective library.