I wish I had useful ideas to offer; I think the model you're currently using is great, and I'd be all over it ... I'm hoping that once I graduate from my masters program I'll be able to start signing up again! (It's not the time commitment for reccing that I can't manage; it's the fact that I never get to spend time browsing for fanart anymore!)

I wonder if some combination of the crack_van-esque model you have now, and the "free-reccing" people have suggested above, might help? Maybe have people sign up for fandoms each month as before, but also host a free-for-all week where anyone who wants to can post recs in any fandom they like (in posts, not comments)?