Sometimes They Find Trouble. Sometimes Trouble Finds Them

Not every problem gets wrapped up with a neat little bow before the PCs move on. Sometimes the PCs start to interact with a plot line or NPC group in an area, and then end up leaving before things wrap up but not before causing a world of trouble for said NPC. This can happen for a lot of reasons. The PCs think what they're interacting with is tied to a different plot, the PCs get bored in an area, they don't know how to follow up on a thing, or the cost of following up feels higher than the payout, or sometimes something else is just a higher priority. When that happens, one of two things can happen. The plot can return to normal - or as normal as it can get with Hurricane  'The PCs' having just torn right through a chunk of it. Alternatively, the NPCs pushing the plot can go looking for the PCs in much the same way PCs often go looking for the cause of trouble.

Plots left untied coming back to haunt people is a classic staple in a lot of fiction. The tension for an episode, comic, or arc is upped not only because of what the PCs are trying to deal with, but because of interference coming from something they had previously dealt with. It presents the opportunity for two different antagonists to also interact, perhaps finding that they can work well together - or perhaps finding that they want nothing more than to stop the other from getting what they want.

For this reason alone, I highly recommend revisiting old plots and untied off plot threads and figuring out what is going on. It gives a number of advantages for you as the GM as well. The two biggest is that it gives the world a sense of life - things left behind will come looking for the people who made a mess of things which shows that the world doesn't only exist when the PCs a re there - and it saves you prep time because instead of having to build a whole new villain and motivation, you already have those built for you.

The returning villain can be anything from an actual escaped villain, a lieutenant that survived the onslaught, a monster from a random encounter that managed to escape but wants revenge. Almost anything that has had a negative interaction with the PCs and survived can come back. Even better, they've survived an interaction with the PCs so they know something about what the PCs can do.

That Mastermind Thief that  survived the purge of the thieves guild? You think they're not going to make custom Batman Villain-esque plans for individual PCs when they go at them? Even something as simple as a brutish raider will likely at least know that the person in priest robes keeps picking friends up off the floor and brings them back into the fight.

Recurring and returning threats give you a wonderful chance to up the ante and danger of a fight not just with a higher stat block, but with advanced knowledge of the PCs, who they are, how they work, and what they will likely do in a fight. This gives you, as the GM, an invitation to customize an encounter or series of encounters to more purposefully deny favorite tactics of the PCs and see what else they have left.

And you can do more than just attack the PCs in combat encounters with this. How does this particular bit of trouble actually go about getting revenge? The classic villain bit from tropes and cartoons is go after those who helped the PCs. However, to really rile the players up I've found little works better than going after their reputation. PCs don't like it when friends start becoming wary, and places they earned favor for deals are now against them. Obviously this can depend on the group, but remember it also depends on the villain. A charismatic noble may send assassins, but they may also just spread the word through their circle of peers about the brutish theft and mistreatment they suffered at the hands of this group of brigands. Or they may go even further and hire those assassins, but not to go after the PCs but to pretend to be the PCs while doing actions that are then blamed on the PCs. Which reinforces their story, and puts more people against the PCs.

You can have fun with it. And reminding the players that sometimes trouble will find them  can also be a wonderful thing. If nothing else, it is something to think about.