DRPC: A battle-tested, drop-in replacement for gRPC in under 3kloc
drpc.io
drpc.io
Even though gRPC works fine overall, of course a simpler, faster implementation is always welcome.
But having used Capn Proto, I would reach for that rather than Proto if starting clean slate. It's much more flexible and expressive. If your language has good support, that is.
I haven't used any of the two yet though so I can't comment on their differences.
I'd expect such an endeavor to pick something like Google's Flatbuffers, Apache Arrow or something similar. Although to be fair, I'm also guilty of having far more experience with protobuf than other formats.