We'll go with this as an item I think, if it's good by you:
Windblades
A favorite weapon of denizens of the Plane of Air and their Material Plane offspring, these curved slashing blades are connected by a chain, enabling the user to lash out at a distance in a flurry of slashing strikes.
Damage 1d4 Slashing
Finesse, Special, Reach (10'), Versatile (1d6)
Versatile: When attacking an enemy further away than 5 feet, the momentum of this weapon increases the damage potential of this weapon.
Special: Bladestorm - After making an attack with the Windblades, you may use a bonus action to attack with the second blade, dealing 1d4 (1d6) Slashing damage on a hit. You do not add your ability modifier to this damage.
My thought is if you are in melee range with someone, you can do 1d4/1d4 with Attack and Bonus Action offhand attack. If you have the enemy at range, however, the attack(s) get stronger to emphasize the whipping and pulling motion of the reach attack.
What's everyone else think as well? I like the idea that staying out of range keeps him more deadly with the weapon, but having a penalty of sorts for being close in. I know this isn't the normal Versatile use and I'm totally open to other suggestions/thoughts.