[larma]

Which modern serialized protocol do you envision here? That comes with good extensibility by design?

Just the fact that XML is old, doesn't mean it's bad. It might be a bit unnecessary verbose or hard to parse for constrained devices, but luckily there is EXI to solve these issues when efficiency is key.

[artursapek]

JSON? Or optionally something like protocol buffers?

[edhelas]

Ah yes, JSON :3 XMPP has an extension for that https://xmpp.org/extensions/xep-0295.html