Asterion’s story arc could be directed in many directions and fits well with some other classes.

Jaheira must always have some levels in Druid

Minsc can go pretty much any martial class, especially Barbarian.

Laziel can be rather flexible, I can see her taking monk levels, but she needs a fighter base level.

Gale must be a wizard, but I can see a few levels as a cleric of Mystra or Shar.

Halsin must be Druid as his story makes him an Arch Druid and a low level Druid just don’t fit. Maybe a small martial dip…

Mynthera (sp?) can be flexible; however, her story arc is still not known.