if you have caught 60 kinds of pkmn prof oak will update your pokedex to national pokedex and will be able to get the ruby stone that celio asks for.
i think he will give you the rainbow pass to, not sure though. after you give celio the ruby and the sapphire stones, you will be able to go ahead and capture mewtwo at the cerulean cave.
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You get access to four, five, six and seven island :)
Nothing. Don't waste your time.
with skills :>)
Then you beat them 100 times and win :)
See question "Is this a good team to beat the elite four"'s answer.