For "open-sourcing" SwiftUI and Combine, I’ve changed my mind after the huge Concurrency roadmap. I believe it would be easier and most time-efficient to develop the open-source version of Combine and SwiftUI with a full set of concurrency features including async-await and Actors. SwiftUI on Apple platforms may also get new set of APIs using Swift’s concurrency (possibly after WWDC21).
Taking these into consideration, I don’t think it’s the proper time for the community to work on these two projects now. We need to get all the concurrency stuff done first, and wait for SwiftUI to get mature and fit into Swift’s development roadmap.
P.S. Considering that Swift Concurrency is largely pushed by Apple, it may be the key of a huge update to SwiftUI 3. So let’s wait and see what’s going on in WWDC.