This idea for an RPG one-shot is more of a “meta” hook than a specific idea. This is playing off the idea that RPGs are generally about multiple protagonists with specialized skills working together (a la Mission: Impossible) while many examples of genre fiction revolve around a solo hero who can do it all (a la James Bond).
The idea is to set up a one-shot where several solo heroes work together. They aren't necessarily carbon copies of each other, but they all have similar skills sets that enable them to handle their usual sorts of adventures by themselves (or with the occasional sidekick, tech support guy, etc.).
Maybe this means a U.N. mission so delicate that the U.S., Russia, Great Britain, China, and France each insist on sending their best agent. Maybe the world’s greatest detectives all end up on the trail of the same mystery. Maybe we're talking 1930s mystery men --- a teamup would be anachronistic, since this would be pre-Justice Society. Maybe it's a direct-to-video action movie throwdown starring Steven Seagal, Jean Claude Van Damme, Wesley Snipes, and Dolph Lundgren.