Over the course of the core game (not counting DLC) you will gain companions to fill the following roles, which basically cover all the standard choices.
The roles of the companion mages are a bit blurred, because you can hugely change their role with the relevant abilities, but their specialisations become the guide. The other classes of companion are locked to specific weapon types (and should probably only take the expected role for that type).
- Warrior Two Handed or Mage (~Any)
- Warrior Weapon & Shield
- Warrior Two Handed [optional]
- Rogue Archer
- Rogue Dual Wield [optional]
- Mage (~Damage)
- Mage (~Healer)
As such, I'd recommend you play as whatever class you want to be, because you're going to be duplicating a companion whatever happens - and this doesn't really matter, there are few sections of the game where you're required to have a specific companion with you (usually only the companion quests and a few major plot points).