Ok, my hack is more like an expansion to the FireRed game, but I still need to shaken up the plot and add in some side-plots as well.
In my hack, I planned an additional area to explore after you beat the game. Its called "Challenge Mountain". What I had planned for its story was that...
Challenge Mountain is a recent geographical structure that appeared after a very unusual spectacle of lights swirled high up in the skies above the area where Challenge Mountain now is. It is home to extraordinairly strong Pokemon that hail from every region. Scientists have tried to investigate the mountain, but nothing quite seems to explain its appearance. They have decoded relics written in Unown about a "Chosen One". Like the Battle Tower in Johto and the Battle Frontier near Hoenn, it attracts strong trainers from all over the world. It is said that the place just oozes the sense of strength, and word on the block is that many Legendary Pokemon reside there. Of course, this is like cheese to a Rattata for the evil organizations of Team Rocket, but imagine the look on their faces when they see Team Magma and Aqua fight there too! Its one trainers dream to be a Pokemon champion, its another to solve the mystery of Challenge Mountain.
Yeah its kinda corny and not too unique, but its the best I got for what I have just made up now. Its a rough draft really.