Saw some suggestions for one-shots here so I though I'd try one! Like in the Uncanny X-Men title, the setting takes the player to the Blue Area of the Moon but before any action starts, theres two portals. Although both will have the same setting, they'll have distinct bosses. Through one portal you fight alongside the X-Men against the Shiar Imperial Guard to save the life of Jean Grey, and through the other portal, you fight against the X-Men with the Shiar helping you. The final boss will, of course, be Dark Phoenix! Loot, I'd imagine, would be based on X-Men space-stuff like the Shiar and Phoenix, and maybe the Starjammers.
Edit: I'd also like for this imaginary one-shot to have cool RNG-based mechanics where the each side (X-Men and Shiar) has four possible bosses for you to face, but you only actually fight two (as is the norm for one-shots) before fighting the final boss. Bosses might be Storm, Colossus, Wolverine, Cyclops, Nightcrawler, maybe even Jean Grey herself as Marvel Girl before Dark Phoenix rises and Gladiator, Hussar, Oracle... (I can't remember all the members during that time).