Following up on my notion of "cusp" versus "rise" characters, character re-use is central. So now I'm thinking about re-using a character. Obviously, if the character was intended to be re-used, it's easy. Or if the character got away, all you need is a motivation, either a diamond as big as the Ritz, or revenge if the character was an onion-skin away from capture.
Maybe character write-ups should come with a "re-use" category, indicating the ease of incorporating the character into your campaign over a long term. Well, relatively few characters are sold with the idea that they can't be reused (though there are situations that call for it: Dr. Immoral Dilemma, for instance, also known as SAW).
But let's say you have a character who was a one-off but the players are still talking about him or her. How do you bring that villain back?
And, in fact, I'm going to challenge my earlier idea and say that there are some characters who are refreshing to see again but they aren't tied to any long-term goal. They're just wacky or inventive or fun. The Foxbat character from Champions was like this.
Anyway, in one sense, it's a silly question. In my youth, when we wanted to bring a character back, we said, "He broke out of jail and now he wants revenge. And he's bringing friends." And it can be that simple. If that's what you want to do, go for it.
On the other hand, that was hit and miss. Sometimes it was worth doing and sometimes the character just showed up and it was another ho-hum session.
Nowadays, with more time between sessions, I'd ask myself some questions before bringing a character back.
Why was the character popular? Was it the connection with one of the PCs? Was it the cool power? Was it the plan? Was it the hostage situation? The costume? The seasonal connection? The fact that the villain always brings candy canes? You don't want to destroy what made the character a success in the first place. Presumably you're bringing the character back because the players talk about him or her. What do they say?
You don't have to do the exact same thing, of course: sometimes it's fine to riff off the previous adventure, presenting the villain and heroes in the opposite light. Maybe the villain is trying to shake off the depression after having tasted godhood. Maybe the villain wants to be arrested because he's hooked on prison food (The Emerald Epicurean has been promoted at the Superguy prison, and his breakfast souffles are to die for.)
What do you need to change, if anything? When you resolved the last adventure, what had changed for the PCs or the villain? If the villain had a great plan revolving around the worship of Samhain and it's early Easter, do you have to take that into account? If the villain professed her undying attention for one of the heroes who has since gotten publicly married, does that change things? If the villain wanted to get hold of the eighth star of the Pleiades that fell to earth in the fourth century AD, and it got destroyed, what's the villain's plan now?
In fact, that can be a large part of the adventure: a plan that would have been fine before the heroes got responsibility fails because the heroes just have too much to do nowadays (no, I'm not projecting; why do you ask?) so the villain falls on increasingly desperate attempts to get their attentions.
A special case of change is if the villain died. Coming back from the dead is almost as old as comics, dating to when they wanted to bring back the Joker, so there's no shame is revealing that the villain survived the terrible fate that apparently befell them. (Bonus points if someone said, "No one could have survived that!" when he or she died.) Heck, you can even have the villain actually be an imposter trying to duplicate the original's MO, or have the villain simply shrug it off.
"But...I sent you to the pits of Tartarus!"
"I got better."
Why is the villain back? Let's face it, any sensible mercenary villain would go, "Hey, they beat me. I'll go work in the Congo, where there's only war and strife to worry about." So why did the villain come back here, to the campaign location? There are a couple of possibilities. Revenge, of course. Maybe what they want is only available here (there's a reason that the museum of supervillain trophies is in the town with superheroes), or they need the eclipse that's going to be visible locally. Maybe they are just too attracted to the hero or NPC to stay away.
Sometimes it's fun to beat up the bad guy who keeps trying to win (though my players are more likely to find out his inner problem and set him up with a job); sometimes there's a complex scheme that evolves from the return of a bad guy.