Interesting idea, changing FA is definitely a great way to make White Knights more rewarding. I personally do not like the randomness of final attack and have an own idea of a change making it only trigger after the second power strike/slash blast. The point being that it would allow me to practice consistently timing a charged blow (stun) right before a monster magic attacks, adding more thought to the gameplay. This would however lower our damage output on single target frozen mobs and bosses and our damage would have to be slightly buffed to avoid this becoming a nerf.
Allowing FA to trigger another knockback could also make it possible for me to use charged blow more consistently.
Would love the hear a Crusaders opinion on this.