What if equipping abilities and spells cost more action points for cross classing?
for example:
2x the ability points for cross classing under the same discipline (i.e. Gladiator -> Lancer)
and
3x the ability points for cross classing outside your discipline (i.e. Gladiator -> Conjurer)

This might put a limit on how many skills people use outside of their own class, lending a bit more "uniqueness" to each class in their own right, and allowing people to continue to customize their character.