Three things I personally believe, but don't have any proof to back up.... just gut feelings:
Character totals from series will not be going down. So if Wolf or Falco get cut, they will likely be replaced by another Star Fox Rep. I don't know why I think this, especially because the number of Mario reps went down from Melee to Brawl (because of Dr. Mario.) I'm iffy on this one, but I think it makes sense.
Characters with completely unique move sets (except maybe Snake) are safe.
Characters that are the sole representatives of their series are safe.