Playing tank is pointless in this game. Most of the time the AI will just ignore the tank, except if you give him the glass cannon trait. Though that would probably make him being cc'ed all the time, so not that fun to play.
As in most games mages are weak in early game but grow strong in late game. Not sure if mages can outshine physical in late-game though they have some strong source skills I heard. But for most of the first act, damage of mages is pretty low, because you can't boost the damage with great weapons. As far as I remember mages start to feel better after level 7, though our archer still easily outshined our mage.
Even more with Ifans crossbow.
Though as Try2Handing kind of scratched on: A full mage setup will have a hard time finding enough decent weapons, in this case wands. Though you could say going one wand and one shield isn't that bad either though might not fit the typical mage feeling.
Everyone having skills to replenish armor and to teleport enemies around will probably be necessary.