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'