I’m looking forward to this!
I’ve cursed forum software for years and a recognized Stackexchange and Quora as important, thoughtful, successful attempts to do something about the problem. I am glad to see that you are continuing to iterate on the problem, and even happier that it is open source.
I think though that Wan raises an excellent point about the licensing though. I can appreciate the desire to be able to re-license, but I hope you can appreciate why potential contributors would be reluctant to participate. I’ll also point out that there is nothing to stop people from creating a fork right now and accepting commits without the encumbrance of the licensing agreement.
Wordpress/Automattic managed to pull-off its achievement without having a paid staff and a paid product offering in place until well after the open source project took off. So, you are in many ways starting off way ahead of where Automattic started in being able to capture value from Discourse. You could quickly loose that leading position and favored momentum if the contributor agreement drives an early fork.
Actually, the more I think about it, the more obvious it is to me that the contributor agreement is almost a moot point. You’ll never be able to incorporate contributions from a fork without nullifying the flexibility of the contributor agreement, but any fork will be able to pull anything you publish to your repo.