hi Heiko,
welcome aboard 
in fact, we’re also thinking about webrtc in various ways, how they can be used and shared in a decentral way, and, of course, audio and video calls and file sharing. routing messages over this channel also sounds interesting, indeed.
currently, there is just no webrtc in delta chat at all, so every progress and contributions on this is welcome
at least for the ui, the signal-messenger (also open source) might be a good start at least for android.
best,
björn