What will help with class uniqueness is enforcing the Armory System. The Armory System basically states when you use Weapon "A", you are Class "X". So why allow characters who are not currently Class "X" to use weaponskills that belong to Weapon "A"? Restricting weaponskills (and only weaponskills) to players who are actually using that weapon will go along way to enforcing class uniqueness, I think. Abilities, traits, and spells should remain unrestricted and cross-classable, though.