Buy a new game and complete it.
You have to beat dawn in a Pokemon battle.
You can explore the entire Battle area. You can catch many legendaries. You can make a decent team and fight on WI-FI You can try to fill your national dex. You can go to the underground to play with your friends.... and much more
Battle before the national dex - 52 Battle after the national dex - 68
in Pokemon diamond you can only use it in battle since your cave/house/thing is underground
You pretty much fill the pokedex, battle, trade, and chat with your friends, and play in the underground.
collect all the Pokemon in the national pokedex
have the national dex and go to the battle groud
you have to get a national dex.
no they don't add Pokemon into your pokedex there
YES.
Hopefully this is all correct but, You have to beat the Elite Four, win a master rank contest, capture 50 flags underground, complete the National Pokedex (all 493 pokemon.), and last but not least get a 100 win streak at the Battle Tower.
To get stars on Pokemon platinum here is what you have to do... get 50 flags underground win 100 battles in a row at the battle tower beat the Pokemon league 10 times