Emacs has excellent Telegram and IRC clients, and is definitely endorsed by the FSF. In this vein, a deltachat-emacs would be just great.
I use emacs for email, but it my GNUS reads the local inbox that is populated by GetMail every 5 minutes. This works great in general, but I would like to use it with Delta Chat as well (something as grand as Telega is not expected), which would mean having an IM-level of responsiveness for wherever I have my chat set up. DeltaChat is FSF, too, so I have hopes; but nothing seems evident at EmacsWiki: Category Chat Client.
Requests and responses are sent as simple JSON lines without Content-Length prefixes, so built-in JSON-RPC support will not work. It should still be easy to generate JSON messages yourself with built-in JSON library or adapt deltachat-rpc-server to speak LSP-like protocol with Context-Length prefixes.
I do not plan to write an Emacs client, but if someone wants to do it, the RPC server part is done already.