A group of famous heroes/adventurers are summoned before a great council of races and charged with a mission of almighty importance. The entire world is under threat Outsiders, creatures from beyond the Outer Gates, beyond the realms of mortal reality are trying to consume the world. The heroes gave saved the world time and again and so the council charge them to do it again and the heroes accept.
Thus starts an epic adventure to discover the cause and source of the Outsiders and to find a way to stop them.
However, the twist is that the Outsiders are no Outsiders at all. They are a games development company and their developers/designers/writers are coming up with a world identical to theirs. The Multiverse is awash with universes born from every whim of imagination that will ever be but there can only ever be one of each, so when the actions of the games company converge with the reality of the heroes universe, strange things start to happen.
Slowly their world is being transformed to obey the rules designed by the game designers but also the developers see weird bugs and code fragments appearing, strange bodies of code and weird events on their servers they never designed whilst they are building their product.
Eventually, the heroes becomes aware of the Outsiders and a game designer becomes aware of the reality they are co-opting for their own profit. Together they work to save their world.
Lots of possibilities for romance, adventure, office politics, corporate sabotage. Two worlds clash, etc. The effects on countless cultures by artificial rules designed to enhance players fun becoming like laws of physics.
Perhaps the game designer originally designed the world exactly as theirs but as changes came down from management and focus groups to increase fun and monetisation the 'real' world began to suffer. Game designer perhaps needs to drastically change the game design so much it severs the two worlds and allows them to co-exist, but he'll never see/hear from the people of that world again (maybe he falls in love with one, manages to go there as an in-game Outsider bad-guy faction person).