Well I can edit what people say in the game...
I have managed to change all the trainer sprites to the DS ones in Fire red... some tileset editing... I can change the pokemon evolutions from trade to levels... I can edit overworld sprites...
Not very good at scripting... I cant script. I can change some sprites... Its difficult Most need you to change pallets :P
I can edit the Logo of Gamefreak to display a custom logo.
I can edit the Start up screen that displays Nintendo, Pokemon, creatures Inc.
I can edit the Gamefreak logo on the title screen below the PRESS START.
I can change where pokemon appear and what level they will be.
I can edit trainers to have different pokemon or a double battle instead...
The only script i can write is to make someone give you the NATIONAL DEX.