Do you have any concept art or anything drawn out or some sort of story skeleton? Are there any particular archetypes you want to see in the game? sometimes when you're stuck it helps to type cast your characters. You're not gonna get a lot of usable ideas if the only information you're providing is an arbitrary number of fighters with a vague goal. You could try working backwards, come up with the villain's story and circumstances, creating characters that would logically fit. The vast majority of classic fighting games rely on the tried and true tournament story where the final boss is a person of power that
A. is the tournaments super champion
B. Is some kind of crime boss someone wants revenge on
C. Is some crazy terrorist
D. Kidnapped a butt load of people slapped them on and island and forced them to fight
E. Promising to grant a wish upon victory
Then there are the franchise vs franchise games, where you're pitted against heroes and villains from opposing franchises, ultimately culminating in a battle against a mega villain alla green laughing slime guy, onslaught, galactus, etc.
Just remember to keep it simple, people aren't playing a fighting game for a convoluted story, unless you want to make a game like no more heroes haha