And here's another one! A much smaller and quicker idea this time.
The Shadow City
(Inspired by Destiny 2: Lightfall, Army of Two: The 40th Day, The Division 2: Warlords of New York, and Schrodinger's Heist by AnonymousTwit)
One night, when the clock ticks to midnight, the futuristic city of Morband, within the greater city of Tokyo, is invaded by an unknown enemy.
No one knows who they are, no one knows where they came from, no one knows how they even got into the city. One second the city was at peace, the next there was an army of armed men and women in black garbs and unmarked uniforms patrolling the streets in military vehicles, setting up checkpoint across the city entrances and intersections, and putting the entire city under martial law. Most of the heroes in the city turn themselves over to the invaders' side. The rest are forced underground and are hunted like dogs.
The invaders are called the Shadow Legion, and they have placed the entirety of Morband under their control.
And now Class A are trapped inside.
They had been off on a trip to Morband to explore the sights and see the new technologies that would be available to heroes soon, but had been cut off from each other and left scattered over the city in groups or on their own as the Shadow Legion invaded, forced to watch as the soldiers in black uniforms gun down anyone on the streets - mostly villains - force civilians back into their homes, and hunt down any of the heroes that have refused to side with them.
The invasion of Morband evolves into a national crisis, more so than that of All Might's retirement, and every hero in the immediate vicinity of the city, as well as from across Japan as well, are called into action. The Japanese Self Defence Force is mobilised for the first time in decades, and everyone that the government could muster was deployed in the face of this invasion.
And meanwhile, the scattered students within the city are forced to take shelter where they can, away from the eyes of the Shadow Legion as they scour the city. Some of them choose to hide away, unwilling to fight against the armed force for a multitude of reasons. Others decided to fight the new enemy, linking up with local resistance movements, hero, villain, civilian, and vigilante alike, to batter the Shadow Legion and their hero allies as much as they can until help can arrive, and maybe even force them to flee the city.
However, as the Shadow Legion's invasion of the city slowly turns into a hostage situation for the entirety of Morband, the students must figure out what this army of villains wants before they kill anyone else.
For there is something in Morband, something buried underneath Morband. Something dark. Something cold. Something that's been waiting to be freed for years...
And now that something is ready to be released.