I dunno but I really don't like that one levels only skills and weapon lines on the active bar the moment when getting exp ticks (by killing an enemy, finishing an activity or handing in a quest). I often happened to be on the "wrong" bar at that specific moment leading to my chars that I am leveling up to have much higher levels for front bar things than on back bar things and it feels awkward to create an artificial build to catch up with my back bar things.
I understand that if exp is divided between active and passive bar many players will be dissatisfied when they are out to specifically drive the level of a certain thing the same way as one is forced to do today.
I also understand one cannot just apply the same exp on active and passive bar unless one wants to accelerate leveling for players.
Ideally, I imagine, some division could occur in case bar is swapped during a fight.
Other ideas?