Obviously this is all going to be highly dependent on the environment and the objective.
So perhaps you shouldn’t think in 8 man setups, but more in therms of couples or trios being effective together.
A heavy engineer with an upgraded heavy weapon while a light covert supplies him with information can be a nice search and destroy couple who can do surgical strikes on the enemy.
A heavy soldier and medic are a brilliant ‘walking supply crate’ being the backbone for the rest of the team.
Any class medium/heavy paired with a light medic could be great for surprise objective steals.
This is just possibilities, I have no idea how this will turn out in reality, but I imagine that in ways like described above, two or three friends grouped up together can do tremendous things.