I can send OMEMO messages between XMPP and IRC. But that's besides the point. My comment about having one standard was about not having one for DM's and another for group chats, when everyone is using only one (because group chats serves for DM's).

I am not sure if it's possible (or desirable). Usually there are specific features (or simplifications) on each of them to warn a separate protocol. For instance, if you are doing a private zap, you want the amount to be public and the rest private, which creates metadata leaks on purpose. Sometimes, "leaks" are desirable. In other times they must be forbidden. But who knows, maybe there is a way to merge them all.