just go crazy catching Pokemon and raise or trade them. or you could restart your game and play it again!!!!
or raise your Pokemon more and challenge the elite four again
or catch mewtwo
or go talk to proff. oak, then celio on one island and start the national dex quest
go crazy and lvl up your Pokemon to lvl100 witha party of your six favorite Pokemon
well there are a few things after beating the elite four in leaf green/fire red, normally a guard would be standing in front of Cereleun Cave when you beat the elite four it is possible to get in the cave since the guard is gone, then you can catch MewTwo, it's pretty fun and simple, you can also find legendary dogs running in the grasses of Kanto after beating the Elite Four so it is actually a rare type and after that you can just play the game as it was before.
By beating the elite for then Beating the champion.
Beat them and the champion
Prof oak after beating the elite four.
I think by beating the Elite Four
This is an upgraded version of the pokedex you will get this after becoming pokmeon champion as long as you recorded 60 species of Pokemon in the pokedex.
The Pokemon League after beating the Elite 4.
after beating the elite four you'd get rainbow pass
Try to complete the Pokédex.
Defeat the elite four and become Pokemon champion with 60 Pokemon recorded in your pokedex.
When youre versing the champion
After beating the elite 4 he/she randomly appears in the wild so you have to find him/her
yes but first you need to get the national pokedex from oak after beating the elite four.