There are a couple of different options available, as I thought a more prompt-based option might make a nice change. For this, it would need people who have enough stories* to sign up to have their stories remixed. Then people who want to write, for which there would be no qualification, but they'd give some preferences about who they want to remix. Then I'd assign everyone so that the number of remixers assigned to each individual author is proportional to the number of qualifying stories they have.
* 5 stories of at least 500 words that are not co-authored, remixes, works in progress or crossovers with non-Doctor Who fandoms
Are you interested in another round of Doctor Who Remix?
Yes, exchange based (as last year)
Yes, prompt based (as described above)
Yes, other (will explain in comments)