Over the weekend I finally managed to see Captain America: Civil War. There won't be any spoilers in this post for the movie, but it got me thinking. In the trailers for Civil War, and the basis behind the story, is there is a disagreement on how to go forward based on the collateral damage of the past. It's one of the reasons I think that Marvel is beating DC in the movie game - Marvel isn't afraid to explore the consequences of their earlier movies - and it is something that you can use as a GM. Do it right, and you can end up with your game writing its own next arc. Let's talk about that today.
The Spider and the Butterfly
In the anime Trigun there is a flashback where the main character wants to free a butterfly from a spider web. The MC's brother solves the problem by killing the spider, which upsets the MC. He wanted to save the butterfly and not kill the spider. His brother points out that in both ways the spider ends up dead - either by direct action or starving - and that you can't save the butterfly without hurting the spider, or the spider without hurting the butterfly.
In other words, while every mushroom cloud may have its silver lining, there is always someone harmed from the consequences of a good deed. (and yes, I know I used absolutes. It's philosophy, there are exceptions to every rule ;) )
The idea here is to explore the collateral damage. What happened that maybe didn't get focused on and could spawn your next villain or plot arc?
Did the PCs depose an Evil King? Said Evil King probably had allies who are not too thrilled. That corrupt wizard could have an evil apprentice or child. The next ninja master of the Shadow Clan may need to avenge the old one before he or she can take full control of the clan.
Those are basic ones, and boil down to having another 'evil' character show up. Where things can really get fun though is when sympathetic villains show up. For example: the only survivor of the town that got wiped out while the PCs were fighting said evil king or wizard? What if they wanted revenge on the people who said they'd help but didn't do it in time?
You can go further too. Someone who lost a friend/relative/lover in the fire caused by an errant fireball. The merchant's daughter whose father/mother was killed because the PCs chased the trolls out from under the bridge but didn't follow through and finish up. A young warrior looking to reclaim their father's sword, that just so happens to be the +1 longsword resting on the hip of the fighter - found deep in the dungeon.
Add To The Story
The point with this is that there are elements to the stories you've already told in your game that haven't been touched on, so bring them into the fore. Bring the world to life. When someone comes looking for the sword a PC found in a dungeon it adds life to the world. That sword now has a story, and that story is in the game.
The same can be done for NPCs and villains. Adding depth to characters post humously can give the world more of a sense of life too. Someone who knew the corrupt wizard when he was actually a good person, or knew the stakes he was playing on, can do a lot fo change the perspective of the character. It doesn't change that he/she was evil and the PCs killed them, but how they fell can be a good tale, and also warn the PCs that they could face the same fate.
One thing to warn about is to not use this to punish the characters. For example, if the PCs clear an evil dungeon - unless it is specified beforehand as being a thing - don't add that the dungeon also produced a spore the locals used to cure some crazy disease that now runs rampant. The idea isn't to make the PCs feel horrible about things they had no control over or way of knowing, but to add complexity to the world.
The person coming to avenge the Evil King shouldn't be full of stories of how the king saved puppies and never harmed a fly and really the PCs had it all wrong. Don't make a right into a wrong. Don't make a good act into an evil act. Explore consequences. Add depths. Add shades of gray.
And above all, keep to the themes of your game.