Once you discover that map design in TF2 is all about carefully managing stalemates, you'll learn that the "fun quantity" is entirely dependent on how unique you can make the geometry while giving each class the capability to do their job well.
There's really no special rules for A/D that don't apply to other game modes.
Give players a place to fight over for 2-4 minutes, resolve the situation, and motivate them to move to the next area.