Coming a bit late to the party, but I’m very interested in this (as part of this roadmap item in particular). The remote build system is in need of a fresh bowl of air imho, and a new protocol could bring a lot (if only by switching to a pull model rather that a push one. Please someone do that!).
For the protocol itself, I’d have a strong preference for REAPI, if only because it’s the closest thing to a standard for remote builds (bringing amongst other things a number of great server implementation and hosted services). And also because (based on some high-level discussions and readings, I could never find the time to look at it closely enough) it’s very nicely designed with performance and extensibility in mind, making it potentially a very good fit for Nix.
Regarding syndicated, I’m curious how it behaves latency-wise. The “conversational” aspect makes it sound like the transactions could take a non-trivial amount of time (which is already an issue with the current protocol afaict). I wonder whether there’s some cleverness to keep things fast