I take a very broad definition of AU, and have used it in terms of alt-canon (a left turn, like you said above) but generally I mean replacing the setting of Dragon Age. Of course, that can entail a complete alt-canon or it can be basically a novelisation set in a different universe.

So... examples: Coffee, Black is a coffee-shop AU, but it is so far removed from canon that it's basically original fiction using someone else's characters. (I still enjoyed it though. It's interesting to speculate on the different relationships that people may have had in another place, another time.)

Jazz Age, meanwhile, is a 1920s AU that sticks pretty darn closely to the canon, albeit with necessary AU adjustments. I really liked this, I thought it was very clever, and the AU adjustments put a different spin on the events (plus the protagonist is great, lots of his thoughts, very believable relationships, loved it).

Then again, I call an AU a situation where both Bethany and Carver lived, but the setting/events are still the same.... which would also be really interesting for exploring their dynamics.

I think all of the different types of AU can do something really interesting, although the first kind - the completely alt-canon, AU - is definitely the closest to original fiction (and also the least likely to encode meta on the DA universe).