One think we need to think early on is whether we should support multiple content licenses, that is, different posts being under different licenses.
While the choice of license can be done right before actually creating content, the support for multiple licenses needs to be in the code, and therefore needs to be decided early on.
One place where this may crop up early is if we want to import both old and new posts from SE, while complying to the original licenses the posts were provided under.
Another way it could crop up is when importing content from several places (say SE on one hand, and a currently separate site that wants to migrate to us on the other hand), which may have different licenses on their existing content.
Finally, we might decide on a different license for newly created stuff on our site (and be it only in the addition of a “version X or later” clause), in which case we’d also have to keep track on the license differences between imported and newly written stuff.