This is a great post (and I'm late reading it, of course). I think AUs can tell more about the characters than any other genre of fanfiction, because the world we're used to them living in is either gone completely or changed in some way. When that happens the characters stand out more, become more clearer, because the clutter of the ordinary is stripped away.

I have two AUs in the works. One would be a Pivotal Event AU, since it's the Fraser-went-with-Victoria premise (it's also kidfic. Yeah, I know.) The voices in this kind of AU aren't the hard part, it's the jockeying of events and timelines to fit the plot and still be plausible. The hard thing for me in this one is that the fic is F/K, so I have to figure out how to work Kowalski into it. I'm thinking that Ray V. went undercover just like he did in canon, and Ray K. replaced him. My problem is I don't think linear very well and I'm going to have to do a LOT of outlining, which is something I'm not used to doing.

The other AU I'm working on (and 'working on' means 'just starting to seriously research') keeps the names and faces but changes the era and setting. The universe involved is an amalgam of the storylines in two folk songs: the traditional "Irish Rover" and Stan Rogers' "Barrett's Privateers". The whole thing started as late-night cracked-out LJ commenting (basically "the end of BP is way too sad and needs a fix-it"), but now I really, really want to write it. This one's the most challenging of all for me, because I have to make up a whole new scenario and still keep the characters true. There's also a boatload of research because of the actual historical nature of the thing.

mickeymvt and I are looking into doing a role-reversal AU where Fraser's the cop and Kowalski's the Mountie *see above comment*, because it's seldom if ever been done and we wonder why. We figure we might find out if we actually attempt to write it.