Rose's return in series 4 totally nuked the impact her original exit had and they should have killed her off outright,the Dalek could have shot her instead of the Doctor as they ran towards each other.
She could have then died in the Doctors arms after being dragged onto the TARDIS and the episode would have ended with the good guys totally demoralised setting up the finale where the Doctor had to get everyone to fight on so that Rose's death wasn't for nothing.
But characters rarely die or depart for good in Nu-Who so I suppose it was too much to expect that Donna would stay gone for good as well.
She could have then died in the Doctors arms after being dragged onto the TARDIS and the episode would have ended with the good guys totally demoralised setting up the finale where the Doctor had to get everyone to fight on so that Rose's death wasn't for nothing.
But characters rarely die or depart for good in Nu-Who so I suppose it was too much to expect that Donna would stay gone for good as well.