Forum OpenACS Development: State of XML-RPC, SOAP, etc. for Tcl, AOLserver, and/or OpenACS
Ideally, someone would look at all those, pick the most promising design, and then enhance it until it all the others evaporate beneath the scorching rays of its brilliance and obvious superiority. AKA, I am largely in favor of one XML-RPC package to rule them all (and in the darkenss bind them).
Failing that, it would still be quite helpful if someone would review and evaluate all the existing RPC projects, reporting on their completeness, quality of design and implementation, which would seem to provide the best foundation for future work, etc.
There are, incidentally, large old Tcl-based RPC projects (and other IPC methods) which long predate XML, notably Tcl-DP. (See also dp_RPC.) The obsessively thorough might, at least in theory, also want to review those for design ideas, possible synergies, etc.