As far as AUs go, our largest and broadest baseline is that one version needs to actually differ from the other in some way. This could be from a completely different background (ALIEN WIZARDS from Jupiter), or events in canon having changed and altered some form of their development or the direction their life took. As long as the characters are recognizably themselves within the parameters set by the AU, then we don't mind how out there they get, either.

It's kind of the same thing for Mass Effect. We're not as up on the games as we probably should be, but for a game like that (such as Mass Effect, Dragon Age or even KotOR), it really depends on which "branch" of the story our current version has taken. In Mass Effect's case, I believe a Renegade Shepard versus a Paragon one would be more than sufficient enough to count as an AU.