Man, they don't have any rules for this crap - if they want to do it, they're gonna do it, and it could be literally anything. All three characters so far have been complete surprises for Pass 2. Aint no one thought Min-Min, Steve, and Sephiroth were getting in, they weren't on anyone's minds except their fans.
Granted, Steve had a very large fan base, but be that as it may - my point is these three were big surprises for the fan base at large. It's practically a lottery - and what I mean is, clearly Nintendo knows what it wants, and they make these decisions with intent, but we, the fans, have no idea how to gauge Nintendo's decision making process, so we might as well throw darts when speculating characters. : P
I'm happy Sephiroth got in, I think he's a great inclusion, albeit not one I ever expected like so many others.