About protocols in this vicinity, I've been noticing a missing piece in OSS around transport as well. In Python, you often need incompatible dependency sets in one app, and the usual choices are either ad-hoc subprocess RPC that gets messy over time or HTTP / containers that are overkill and make you change deployment strategy.

I ended up building a protocol for my own use around a very strict subprocess boundary for Python (initially at least, protocol is meant to be universal). It has explicit payload shape, timeout and error semantics. I already went a little too far beyond my usecase with deterministic canonicalization for some common pitfall data types (I think pickle users would understand, though). It still needs some documentation polish, but if anyone would actually use it, I can document it properly and publish it.