You go on to the Sevii Islands. There you find some of the Pokemon from other games. You can also rematch the Elite Four, but they go from being about lvl50 to about lvl65, so you'd need more training first.
I think Team Rocket have something to do with the Sevii Islands as well, but I'm not sure.
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You get access to four, five, six and seven island :)
Its best if you have 1 fire Pokemon and 1 ground Pokemon.
use cheat codes
See question "Is this a good team to beat the elite four"'s answer.