For starters, you can get Mewtwo. You can also get the items to trade with Ruby and Saphire.
There is more, but I can't remember the rest.
One island.
The same you can do in Fire Red.Read my answer to that here:What_can_you_do_after_you_beat_the_elite_four_in_pokemon_fire_red
Fire and water are the best combination to beat the elite 4.
See question "Is this a good team to beat the elite four"'s answer.
You have to migrate them, which I think can only be done when you beat the elite four.
Its best if you have 1 fire Pokemon and 1 ground Pokemon.
You get access to four, five, six and seven island :)
against the elite four infernape will do very good with its fire type moves but since the first gym is ground it is not so easy to beat it with a fire type. i beat the elite four 6 times with my infernape. the beat moves for it are fire blast and flame wheel
first you need the rubys in fire red but first beat the elite four
Nothing. Don't waste your time.
The first thing you have to be sure of is that you have beaten the Elite Four. Do that first!
there are no specific Pokemon that can automatically beat the elite four. HOWEVER, ( i speak from experience) i beat the elite four with one Pokemon whitch was Samurott. i suggest using Samurott 2. hope i helped! :) - Raven