Make 
a melee only item and replace the ranged version with Gauntlets of the Ruined King:
+50 AD
+10% lifesteal
+30%AS
UNIQUE Passive:
Your basic attacks deal true damage equal to 10% of your target's remaining health
Kog'Maw with this item kills everyone in about three seconds.
Other ADCs would do pretty obscene damage with it, too, so that pretty much turns anyone able to use this even remotely reasonably into a game breaker.
the unique passive's juuuuust a little Over Powered. Try "After two melee attacks does 4% of the enemy's health (Has 6 second cooldown on the same enemy)" (NOT USEABLE BY RANGED)
