The Gameverse is a universe formed from every Nintendo series. Every universe is linked to the Gameverse, allowing the gameverse to be used for quick travel.
Super Smash Bros.
The existence of other universes besides their own is unknown to the people of the Mushroom Kingdom, Hyrule, Dream Land, the Pokeverse, and many others. One day, a single being known as the Master Hand wanted each and every universe to see each other. So, he decided to have a fighting tournament, with people from every universe participating. It was held in Master Hand's domain, the World of Trophies.
Subspace, an alternate dimension, was most powerful in the World of Trophies, and they once decided to sabotage the annual fighting. Tabuu, ruler of Subspace, chose a champion to do just this: Metal Mario, Mario's Subspace counterpart. Metal Mario was given a device that could modify fighters in bizarre ways, such as making them giant or metal. Metal Mario utilized this device, (this explains Classic in SSB) but was defeated by several fighters.
Metal Mario, however, survived, and continued experimenting with the device. He somehow created a clone of Bowser: Giga Bowser. Giga Bowser, however, was merely a monster with no intelligence whatsoever, and escaped. He was hunted down by Master Hand's fighters, and Bowser learned how to become Giga Bowser through this. Metal Mario was defeated again, and he fell down into the abyss below the platform he was fought on.
After that, Tabuu gave up on infiltrating the fighting, and decided to go for Plan B: Merging the worlds.