just try to abuse the holes that will exist in their team. if a scout and a roamer are dead, then a hole will exist on the flank. if pocket or demo is dead, then a hole will exist in choke. you should be taking ground that isn't being held and forcing the enemy team to either commit more resources to fight you (freeing up other areas of the map) or forcing them to concede an area and back up/out

Are you asking how to play off of player advantages? Based on what players are dead?

scout - you can jump more
soldier - you can not get forced more
demo - you can take ground more
medic - you can expect them to go for your med more

pyro - you can not get gayed more
heavy - you can not get gayed more
sniper - you can not get gayed more
engi - you can not get gayed more

EDIT
spy - you can not get gayed more

thx kirby

If demo is down on last, no stickies on point - play the point.

what about spy hooli

Freaking hooli...

Well said.

It really depends on the situation. Think what that class is doing for the team at that point and think how you can take ground based off of that- e.g. a demo pick can make pushing without having to use uber a lot easier as there won't be sticky traps, a soldier pick means there's less spam allowing for off uber pushes. Probably the one that changes the least is a med pick. Assuming you have player advantage with a med pick try to take ground slowly and have players, especially your scouts play close to your med so you don't get forced.
In all of these situations I mentioned, I'm considering a 6v6 stalemate where you get a pick to make it 6v5, generally games are a lot more complicated and the same class getting picked isn't something you should always react to in the same way.