Given the problem with licensing and the variety of sites on the SE network, why don't you allow each site to license content in the way that is most appropriate for them? For example, Code Review could continue with the current license, while a religious or language site could use a license appropriate for their content. The license could be linked in the help center for ease of community reference.
There are many problems with this.
First, migration would have to be totally removed from the system. No one other than the content creator can relicense their content. Since every site may have a different license, migration would have to be restricted to sites with the same license or totally removed. It would probably be easier to remove.
Second, it would be confusing to people who participate on multiple sites. I, personally, like to understand the rules for how my contributions can be used before I use a service. I'm sure there are other people like me. If you frustrate these people, they won't use the service at all.
Finally, it would be hard for people who simply want to use the content to understand how to use these different licenses. Plus, the nightmare of exporting the content via data dumps, SE data explorer, and the API - people are allowed to aggregate the remix the content in various ways and it's made easier by the fact that the licensing terms are consistent.
No matter what happens, I do think that the same rules need to be applied across the board.