Just a few ideas I've come up with when playing, btw guys sorry I havn't been on in a while with school etc
Resto Shaman, Frost DK and frost mage/rogue for defense.
Fire Mage with disc priest/resto druid for Flag Carrying
Hunter for controlling middle.
Warrior for pushing.
Usually its a good idea to control mid. If the enemy at mid are nuking the flag carrier when he leaves our building through the tunnel etc. Its the difference between smart midfield control and "the idiots fighting at mid" that don't even realize the flag carrier is running by them.