There does need to be a formal process for adding things and changing things after all, and features need an element of formality of definition in terms of what they do and how they should function - we can't just have dozens of people attempting to hang things from the engine source-tree without any co-ordination or oversight over what is going on. Indeed, I'd like to ensure that people know what they are working on will be accepted before they start working on it. Apart from anything else, being dual licensed means that we take responsibility for all code that is submitted and have to commit to maintaining it thus we need to understand it and its function - its been my experience that commercial customers don't tend to like features being removed after they've been added

[ Note that after Open Language, things become much easier - you'll be able to extend the core without the code for the extensions having to live within the core. Work on the core will then reduce to (essentially) further refactoring, and adding extension hooks for modules to utilise to provide their function ].
Having an entry in the QCC for all features and bug-fixes seems to be a must - it gives a central place that enables us to see easily what is being done [ In particular, bugfixes need to be co-ordinated ] and is perhaps the best place for the 'final' spec for what the feature is.
In regards to discussions...
Github will be restricted to discussions about the code in pull-requests, which isn't really appropriate to have anywhere else - essentially just code-review.
These forums seem to be working quite well in the way we actually intended the engine-contrib mailing list to be, we're going to have an internal chat about this next week now that we have some experience. It might be the mailing list idea dies a death.
At the moment this flow seems quite a good potential flow - chat about it here, come up with proposal, post as enhancement request, questions about implementation here, submit pull-request (at this point there shouldn't be any doubt over its eventual acceptance), code-review type discussions occur on github, code integrated.