Is a Wizard the answer for party balance?

[Herobizkit]

I'm probably crazy, but with two Leaders in a party of six, I'd likely make a third Striker before a Controller. Your DM would thank you for trying to speed up combat.

 

[Argyle King]

Honestly, I'd go with another Warlord. It's not a controller, but -with your current party composition- I believe you could do a lot with another guy who can enforce battlefield control, move people around, and amp up what the rest of the party can do. Go with a different Warlord build than whatever the other guy is doing, so you have a variety of effects. You have a rogue and a ranger for damage, so having two warlords capable of giving out extra attacks, buffs to the party, and debuffs to the enemy should work well.

If you feel that you want more burst and blasts, you could always do a hybrid warlord/wizard and go with the INT build warlord. Otherwise, I'd go with the Psion -as suggested by others. The reason being that (in my opinion) the Psion has good synergy with a Warlord. The Warlord buffs the party, and the psion can debuff the enemy.

 

[mneme]

I'd be tempted to go with a Genasi blaster wizard (or mage). Strikery damage due to Elemental Empowerment, but access to wizard shutdown and at-will bursts.