The questhub quest related to improving Content Negotation in Catalyst continues to be popular, but I've been hesitant to put this in a development cycle quest since I know its a pretty big subject and I'd hate to get it really wrong in core.
I had a bit of time on a few plane rides recent to get a chance to write down all the various thoughts and ideas I've heard around what could be a minimally useful expansion of how Catalyst handles content types beyond classic form data, etc as well as some features to make it easier to use Catalyst for building web services.
These are new core features not intended to compete so much with existing tools like Catalyst::Action::REST but instead to provide some minimal and hopefully correct support out of the box, to help beginners find enough in Catalyst to stick with it. And hopefully better core support will make it easier to improve specific tools (like CAR).
I wrote it up in pod format and put it out on the github organization for any and all to comment, add tickets to, fork and send me corrections, etc.
I don't specifically see this as targeted for the Hamburg release, but most of it I think is straightforward, so I'd be happy to help mentor anyone that wanted to run with it.