Question: what do you do in pokemon diamond after you beat the 7th gym leader?
Answer:  Win the 6th badge.
 Go to the 3rd floor of the Canalave library (north from the Gym)
 Go to Lake Valor (it'll be drained) and defeat the Galactic Commander.
 Go to Lake Verity (near your house) and defeat the Galactic Commander.
 Enter Mt.