The way I've been doing it is to pick any fics the writers especially want comments for -- whether because they didn't get enough in the first go-round or because they want feedback on specific things -- and offer concrit or a synthetic overview or a kind of comparative view of the different fics. It's really up to the writer. I think it's helped me do better beta, and I've had great conversations with the writers along the way. Alas, though, I've been terribly slow about it -- I started last March and have only done about six of the twelve I initially promised, so unless I can pick up speed, it might be awhile. But I love doing it; there's a kind of thrill about looking at how someone writes in that kind of depth. Also, my flist is full of favorite writers, so it's not exactly a hardship. :)

Yep, it's fun to revisit old favorites, too. I spend a lot of time thinking about how stories have changed over the growth of the fandom. You'd probably get a big kick out of it.