Speedsters or Stalls would balance out your team, and seeing as this is clearly an OU team, I'd add Bronzong or Crobat to the mix.
Toxic / Earthquake
Protect / Gyro Ball
Cross Poison / Quick Attack
Heat Wave / Steel Wing / Giga Drain
Confuse Ray / Hypnosis
EDIT: The reason why I chose Bronzong over, say, Probopass or Steelix, and the same with Crobat over Ninjask and Jolteon, is because:
- Bronzong and Crobat both are immuned to Earthquake, whereas Jolteon and Steelix bear x2 weaknesses, and Probopass bears a disappointing x4 weakness.
Ninjask is a good choice, but it's very weak to Stealth Rock, and is really frail.