ATTENTION: There are romhacks for old Pokémon games that let you get every single one in a single save file through balanced changes and that's the coolest shit ever. I'm definitely replaying Fire Red and then Emerald.
Discussion
I seen those pokemon master collectors havks that basically swap the gameshark codes around enabling all wild pokemon from each version to appear in one. I've even seen those hacked versions where they try to merge all the generations into one game. It seems easier to do one the newer versions because the code id for all the monsters is already there. Action replay codes let you force anyone to spawn.