Lots of things. You can get both Hoenn and Kanto starter Pokemon (Hoenn: Torchic, Mudkip, and Treecko. Kanto: Bulbasaur, Charmander, and Squirtle.) You can also catch Groudon. If you have a bud that has HeartGold, ask them for Kyogre, and you can get Rayqaza. The final thing: talk to Blue, and he'll give you the seal case. Hope I helped!:-)
He vanishes.
battle your freinds that's about it :]
after you beat red in Mt.silver
complete the pokedex
beat elite four rematch
in soulsilver you get the red orb in heartgold you get the blue orb
He disappears until you beat the Pokemon league again.
well u can catch the rest of the rares
Earn the national Pokédex, go to the mountain where Red is and beat him.
If you beat trainer red go to Mr Pokemon's house and he will give you a red orb.
beat red and go to prof. oak
You can always catch Groudon. And Rayquaza too.