An idea but no story

[Arkvoodle]

Alright, I've got this idea for a post-apocalyptic game set in a modern setting. The idea is that after the cataclysm (undefined thus far) the world changed and older creatures began to 'wake up'. The concept is that humanity is pretty much extinct and the elder races are returning to reclaim the world. The PCs would be a group of goblins living in an old subway and scavenging to survive.

Anyway, that's pretty much all I've got so far. I'm having trouble turning this idea into a viable game, since it has no real story other than basic survival. I'm probably going to try for an open world/sandbox concept, but I need encounters and adventures to spread around the setting to give players stuff to do. I was hoping to solicit some ideas and suggestions from other DMs to help flesh this game idea out.

Also not 100% on what system to use yet. Leaning toward d20 Modern but I'm not sure.

[Mr.Sandman]

You could try to adapt PF's We Be Goblins trio of adventures. The whole premis is being a tribe of goblins.

[Lorsa]

Sometimes an idea is all you need. Let your players make the story for you.

But yes, you do need to come up with something to interact with in your world. What else is in the subway? Who rules the streets of the city? Is the subway connected to the sewers? Do different subway stations have different tribes or creatures that guard it? Do any of these want to attack/solicit help/interact with the group of goblins that will be portrayed by the players?

When scavenging, what goods can they find? Is some of it useful/dangerous? Perhaps owned/hidden by some other creature? Are there magical artifacts? Who would be interested in buying those from a band of goblins?

If I were you, I would probably have the goblins encounter a lost little girl, equivalent to a starving stray cat, to see what they do with it. Could be interesting, as she probably has different dietary needs than goblins do.

[Jay R]

A. Yes, it's a survival game, at least at first. So run with it. The encounters are scavengers, gangs of thieves, vermin, etc.

B. Eventually, a larger gang from several miles away starts growing and taking over. Then the game becomes either guerrilla warfare against the nascent empire, or joining it and being sent on exploration missions.

[Maglubiyet]

Avariels or raptorans that live in aeries built into decaying skyscrapers.

Mutant storm giants that live in castles atop nuclear fallout dust clouds.

Motorcycle gang human survivors that roam the blasted wastelands outside the cities.

Warforged that worship an AI based on an ancient telecom satellite network. They run factories that manufacture more warforged.

[Karl Aegis]

The dark clouds part to reveal a single ray of sunlight. Descending from this radiant beam is a child, roughly nine years of age, who knows nothing of this world. Boom, raise a child. Good luck.

[Oddman80]

Have you heard of Numenera? (http://www.numenera.com/what-is-numenera/)
it has a similar concept... but it might be more sci-fi than you are interested in...
but the idea of a more primative society living among the remnants of a more advanced society is what the game is built around.

[rafet]

What exists of society? Is there a goblin nation? Or is it all cosmopolitan? Maybe one of them thinks it's time to rebuild goblin society, should they look for ancient texts that describe what a goblin society looked like long times ago? Do they need convince other goblins to join them, and then they fight for a piece of land, and set up trade routes and such?

How do the elder races feel about goblins? Are they kill on sight? Low-lifes that get unnoticed? Maybe they start or join a gang and build a black market network?

Is the world still a mess? Maybe there is a way to undue it? Maybe they were humans, and were mutated into goblins and they are looking to change back?

[Sacrieur]

Characters create the story. Make some characters that want to influence the world.