What are the most balanced 4 v 4 teams you can think of where each class is only used once?

I've been thinking about possible new gamemodes recently, and one I've been looking it is making two small teams completely different to each other.

I think this is part of the attraction of games like Natural Selection, or Starcraft - each side has a fundamentally different set of capabilities so it's really about playing to your strengths.

So say we take the 9 TF2 classes and split them up into two teams of 4 (unfortunately someone has to get left out), is it possible to do this and have balanced teams?

So to start off, I think you'd have to have medic and engie on opposing sides anyway. Maybe splitting up the soldier and demoman is wise also.

After this, I'm at a bit of a loss, so I'd be interested to hear any thoughts you lot might have on how best to do this.

thanks!