Inspired by Lucy, Akira and The Justice League Unlimited episode The Return, this scenario seed centres around a character with constantly evolving powers.
Our superheroes (although it could probably work in other genres) encounter a newly minted hero (it could work with a villain, but I feel the story has more pathos if it involves a fellow 'good guy' - the heroes will, hopefully, be less inclined to simply put a bullet in the character's head) and quickly discover that the character's powers are evolving.
|Justice League Unlimited: The Return|
Depending on how fast you want to make this play out (and the new character's origin story), the character's power levels soon exceed those of the player-characters, but she will continue to get more and more powerful.
At first, the player-characters may think they have a cosmic-level companion to fight their battles for them, but it will soon become evident that this new addition to their universe hasn't got a handle on her escalating abilities.
Do her powers run out of control (à la Akira)? Or does she eventually evolve beyond human comprehension and become something else?
Although this may turn into a pissing contest between the player-characters and the 'supercharged' character, ultimately it will depend on how the player-characters behave once they realise they are dealing with someone way beyond their pay grade.
A key angle to focus on will be the fact that she isn't simply some force of nature like comic books' Galactus, The New Gods or The Celestials, but was once a human being like them and might still retain some degree of humanity.