I believe that the ability to swap the weapons a character is holding for free(no action point) could open a good number of possibilties (for instance, swaping the axe in left hand with the sword in right hand). This could be used to keep a pistol and a melee weapon while being able To use both.
Also, being able To swap between a bow and a sword or à staff and a mace could be fun, i'm suggest that instead of preventing dual wielding with two weapon, great weapon would disallow shields, and would only give their stat and skill bonuses while in main hand