Either a stat boost fruit or something that complements Mihawk's sword style. I don't want it to be something random like Sugar's or Gladius or Moria-- it needs to work with the skills the lil guy already has, which are eyesight, strength, speed, and control.
Plus if he sticks close to Mihawk's original style, then we'll probably get to see Zoro fight him. If it's something wacky, then who knows? Chopper might fight him or something...