Some kind of striking (Muay Thai, boxing/kickboxing, Shotokan/Kyokushin karate or Taekwondo if you can find a really good school) would be good.
Some kind of submission grappling art (Brazillian Jiu Jitsu, Sambo or Catch Wrestling) to supplement your wrestling.
Physical conditioning is always good too, but since you're probably not going to be competing for at least a couple of years anyways I wouldn't really worry too much about it.
You're going to need all of the pieces of the puzzle to fit together at some point, and just as long as you get everything in eventually it doesn't really matter what order it's in.
"I don't want them to like me, because when I'm in the ring, punches fly to the face. It is not a tenderness time. I am not falling in love. And he knows that when the fight starts I am kicking faces."
- Wanderlei Silva