I haven't started reading any of the fic, but I've gotten the impression from discussion posts that there's two contrasting tendencies: being unshackled from established canon and starting with a near-blank slate, and trying to work through the precise ramifications and implications of the AU on established canon. I wouldn't expect the latter to generate as much fic as quickly as the former approach, but the stories that do come out of that working through will probably be more influential in shaping fanon for everybody.

I was thinking that in between Merlin and Star Trek, there was a flurry of infatuation with Leverage, which was also a short season by U.S. broadcast network standards. Maybe we'll ultimately end up with seaasonal fandoms that flourish for three months at a time each year?