Timothy Appnel discusses some ideas for extending the TrackBack specification. There's good stuff there, but backwards-compatibility is a concern; for example, in my trackback implementation for LSblog, the “do I send a trackback ping as a GET or POST” question is basically handled through a hack (does the ping URL use a query string or not). Adding more incompatible changes will increase the complexity of implementations, even if well-intentioned — in particular, moving from RDF to RSS. On the other hand, using the HTTP error handing mechanisms is greatly preferable to the XML-based system that is used now (and which I haven't bothered to implement a SAX parser for yet, because I'm fundamentally lazy).
I also second EngageBrain: if TrackBack is going to be widely adopted, it ought to be written up as an RFC.