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You will able to go to Sevii and Hoenn once you have won the League.
There will be a warp where Groudon used to be, after you have beaten him. That will teleport you to your house in Littleroot. :)
As for the Legendaries there are different ways. Deoxys will be found where the entrance of Dotted Hole is. :D And Ho-Oh and Lugia are controlled by Magma. I can't tell you the area to find these, though. :D