Here's an idea: Weapon mastery.

The weapon mastery skill tree, once unlocked, would contain passives that apply when using specific weapon combinations on each weapon bar.

Some possible examples:

-When using dual wield on one action bar, and two handed on the other action bar, you gain bonuses to the abilities in those weapon trees.
With dual wield bar active, your light and heavy attacks cause splash damage to nearby enemies.
With your 2handed bar active, your attack speed is greatly increased.

-You could take it a step further with synergizing abilities:
If you have whirlwind on your dual wield bar, and cleave on your two handed bar, your whirlwind will now cause bleed damage and your cleave will now hit all enemies in a 360 degree radius.
If you have Uppercut on your two handed bar and twin slashes on your dual wield bar, your uppercut will no longer have a cast time and will strike twice (for less damage with each hit), and your twin slashes will knock the opponent down.


As another example, the weapon mastery bar could allow you to equip a two handed weapon and a shield at the same time. Doing so would only allow you to use 1h+shield active abilities, but you would be granted the passive bonuses from both trees, so your attacks could cause splash damage or have a 100% chance to apply status effects.


The weapon mastery tree could also offer different passive bonuses based on the individual type of weapon you're using. For example using swords or daggers would provide a damage bonus, using maces and mauls would disorient/daze/concuss enemies, and using axes and battleaxes would reduce healing or cause additional splash damage.