Want a cool new idea? Want to introduce a cool new mastermind bad guy but you can’t just have him appear out of no where? Think back to a couple of previous missions, better to use ones that your players might not remember as well (maybe because they are from a long time ago). The best ones are the ones where the players didn’t really get to the bottom of your real plans. Now, you just change the background.
Here’s the idea - OK, last year you set up this whole story line about slavers, but your players got all sword happy and killed everyone before they realized which one was the leader. They hacked him down along with the rabble and barely noticed he had better stats. Now you introduce the mysterious money man (bad guy) and he tells the players how upset he was when they busted up his slavery operations last year. See - now you have a new big bad as, and you built him a background that already makes him your players’ enemy. (Well, and him their enemy.)
It’s normally called ret-con: retroactive continuity. It’s like when they made up all this stupid stuff about Spiderman’s parents being spies. No one planned that when they started the story, but they thought it was a good idea later on. Please come up with something better than long lost parents being spies! You can always add stuff on to someone’s background. The players are seldom great at building full backgrounds, but adding to their background is #1 kind of a cheesy because it’s their background (you didn’t write it, and they often get territorial) and #2 the easy way out. By rewriting your own missions, you have far greater freedom and knowledge.
I’m a big fan of the “permanent” enemy. Now, if you never had an enemy that kept coming back, you can build one. Mr. New Bad Guy can now be really upset that your characters destroyed his slave operation and those bandits and that thief who stole the art work. You know you can hit it the other way too. The players think they were hired by a scared old lady, a sheriff and a knight, but in fact they were being manipulated by the powerful high priest who was testing the team to determine if they were both made of the right stuff and of the proper moral fiber. In fact he has controlled a major part of their adventuring career. It might upset them and make them feel like patsies, but it should still impress them, and there was no chance of them detecting the deceit earlier, because you just thought of it now.
Why? Why do this? Why make the adventures link together in mysterious and over blown ways? Because it is a campaign and not a series of unrelated “module” adventures. Because getting your players angry about what is happening to their characters gets them invested, invested in a manner in which they want to keep coming back to game sessions time and time again. Besides - It’s fun!