You get the chance to encounter Kyogre and Groudon, also the battle frontier is unlocked for you and each of the Pokemon that have been in your party after defeating the elite four will gain a ribbon for defeating the champion. Hope this helped :)
you need the national. dex and need to have beaten the elite four. then you migrate regice, regirock and registeel from Pokemon emerald, ruby or sapphire. have them all in your party and go to the temple in snowpoint city.
Beldum can only be obtained after defeating the elite four. Once the elite four have been defeated fly to Mossdeep City and enter the house north-west of the Pokemon centre. Within is a note from Steven and a Pokeball containing Beldum (ensure you have space in your party before collecting Beldum)
In Pokemon Emerald to solve the Sealed Chamber puzzle you need to put Wailord at the top of your party. Relicanth needs to be at the bottom of your party. This order is the reverse of the order needed in Pokemon Ruby and Sapphire.