1. Who would you have as Player Characters? - The First Overlord (Game-wise) with millions of Minions, Boethia & Jyggalag from The Elder Scrolls, and an assortment of morally ambiguous characters from other games, such as Scorpion and The Bard.
2. Who would pull in to play the villains? - The rulers of many a video game universe, eventually getting paired-down to The Kings of Hyrule and The Mushroom Kingdom, Link & the Mario Brothers, Gannon, Bowser, and their respective armies
3. Why would these pixelated prima donnas be teaming up? (The main idea of the plot)
- The Overlord has been ruthlessly exploiting his subjects for years and has all but destroyed the will of the people, when he is told of the existence of many alternate universes of fantasy such as Nirn and the Ninten-iverse. Not happy knowing that he's not actually Overlord of all known land, he gets Gnarl to super-charge the Tower Heart to be able to teleport him anywhere he can conceive.
His initial attacks on Tamriel are easily quelled by the inhabitants of Summerset Isle, and the Oblivion Crisis makes the public forget about him completely.
From there he is introduced to the various Daedric Princes, who agree to team-up with him to take down not only the Empire, but the Aedra as well.
Once you succeed, the plot twists into a mish-mash of dozens of different universes, all at war with each other.
Eventually all of the Universes are conquered by either the Overlord-Daedra Empire or the Nintendo Powers, leading to an epic final act the likes of which dwarf even Imaginationland and all the Crossover events in the history of Comic Books. Link v Scorpion, Mario v Sonic, Rayquaza v Fatalis. Any and every battle between Nintendo's stable and any other companies' characters that aren't nailed-down to one console is possible.