Some things I keep in mind:
- the spice protocol had a number of iterations to fix some races. It would be great not to repeat the same mistakes, and I don't know if VNC have the same flaws or not.
- the spice agent design isn't great: the system agent proxies messages to the active session. It would be nice if the new solution didn't have such a middle-man.
- with wayland, clipboard sharing isn't really possible. Or not in a seamless way at least. Today it kinda works with some X11 compatibility extensions, but this will eventually go away or change behaviour.
- it's not just about clipboard. We would also want to have some kind of drag and drop (even if limited to files like Spice atm). We may want some windowing integration. We may also want to have access to some desktop services: apps, documents etc.. And what's not.
That leads me to think that virtio-serial is not very well suited, as it doesn't allow various services / processes to come and go. I see vsock as a much better alternative. (I also wonder if it handles control flow any better btw)
I think we shoud work on getting the free desktops our best-class support. To me, this means we need to speak the lingua franca, which is DBus. The great thing is that DBus is also equipped to handle services that come and go, handling discovery, introspection etc. Various services are already available. As mentioned earlier, that's what the GNOME desktop will offer for clipboard sharing. There are good chances other desktops will follow if that design works, as it should be easy for them to implement the same service. That means good reuse of existing desktop code. Speaking DBus on Windows, MacOS or Android isn't an issue. However, vsock support may be a bit tricky atm.
Oh and of course, since this is a new daemon, it would be really a shame not to write it in a modern language (hint! ;-).
Hope that helps,