1. The built-in types are already protos. Imo gRPC wouldn't be a good fit - actually will make the system harder to use.
2. Already can be achieved today via kine[0]
3. Couldn't you build this today via regular CNI? Cilium NetworkPolicies and others basically do this already
4,5 probably don't require 2.0 - can be easily added within existing API via KEP (cri-o already does userns configuration based on annotations)