I'd take a whole lot of major devs and throw them all together in a Smash Bros. style game. Since it's Smash Bros, I'd let Nintendo make it, but I'd throw in the following companies as well: Capcom, Konami, Sega, possibly Namco, and definitely Shanghai Alice. Then all we need is an adventure mode like the Subspace Emissary in Brawl and we'd be all set!
If anyone else likes my idea, feel free to flesh out the roster/adventure mode. I really can't be arsed doing that myself.
Edit: Hell, I'd be happy with just Nintendo/TSA. One ultimate showdown I'd love to see: The figurehead of gaming since 1985 vs. the indie girl that keeps on giving. Or in layman's terms, Mario vs. Reimu. Somebody please do this.