I'd really love an XMPP app that supports both Jingle and legacy file transers, stream resumption, message carbons, Multi User Chat (MUC), and Message Archive Management (MAM).

Not sure how easy that would be with Telepathy, because the Telepathy XMPP client that ships with Plasma 4/5 is pretty s#!t in my experience. I don't know if that's the fault of Telepathy for not supporting certain XEPs or the app for not implementing them.

A good model for the "perfect app" is Conversations on Android.