Definitely, you will not be choosing another Warrior - in fact, since the two warriors you've mentioned are Tanks by default you may want to re-spec the other one to be an off-tank or something. Both Templar and Champion classes can be designed to wield Two Hand weapons, so you can take your pick who will be more "offensive"
Some say Templar is great with combos. And Champion remains very sturdy even if it's spec'd for 2H Skills.
So you have a choice of doing Rogue or Mage. A Rogue can be very effective as you can be the one to pick off Mages and Archers while most of your team will be handling all the melee stuff. Vivienne would most certainly move in and out of melee as she is a Knight Enchanter. Given that you have at least 3 people in Melee, you can maximize the effects of Barrier when she dishes it out.
I think any form of Rogue can be effective. This set-up seems to be high on survival. If decide on getting Daggers, you would be in proximity of your tanks or Vivienne for the benefit of their Taunts and Barrier. You will be free to flank enemies left and right. An archer can be used as well, as ranged attacks can be a great opening to encounters. Just be sure to be out of enemy's range once you loose your arrows.
I'm not too familiar with the Spec Trees of the Rogue, but personally I would pick either a Tempest or an Assassin for any of the Weapon Styles. The concept of using traps doesn't sit with me too well but I don't know if the Artificer Tree has great passives.
Of course as per the base trees, you will pretty much investing into your weapon of choice, and you will get key skills in Subterfuge and Sabotage. For better guidance, I would recommend Matth85's Pre-Skyhold Builds. He discusses in length each kind of class for you to be helped with your decision.