Power Creep

Let us talk about ‘power creep’ and what it means. Then let us talk about how to avoid it. Power creep is mostly a problem of long-running series. It happens when one or more recurring characters get too strong during the series, so that they can’t really be challenged by anything any longer. Needless to say that when it happens to your main character or characters, that is bad for your series. Sometimes, power creep seems unavoidable, but in most cases, there are ways around it.

How does power creep happen? Generally speaking, the higher the stakes get in a series, the stronger the characters have to be to face them. Over the course of a long series, stakes will rise considerably and with them the skills the characters have to develop to be able to overcome the conflicts and be victorious.
It is normal for a character to grow during the course of series. As a matter of fact, it would be weird if they didn’t. Characters gain new skills and new friends, they become more powerful, more famous, more important.
The problem arises when they’ve gotten too much influence, too many friends, too many powers. In a series, every conflict and every climax is supposed to be bigger than the last, so the characters have to keep up with the new stakes and threats to make that happen.

Yet, the stakes must rise every time, so must the characters’ powers, right? Not necessarily, as it were.
Climaxes aren’t about rising powers, they are about solving a problem, about resolving the conflict in some way. In some cases, that means that your main character must gain or develop a skill which makes them equal to the antagonist and allows for them to win the final confrontation. In other cases, it might mean finding the MacGuffin or gathering allies who will help you in that confrontation.
If you don’t want to raise your characters’ powers, it is valid to expand their circle of acquaintances instead. Make the story about them all about making new friends, about helping others so they might be helping your main characters out in the end.
If you’d rather they go on an adventure, make it all about finding the MacGuffin (and, perhaps, making some friends on the way, why not?). It’s not always about becoming more powerful or developing new skills.
If that doesn’t work, either, think about expanding a skill they already have. Perhaps they can hold their breath for ten minutes, but that strange underwater passage demands they hold it for twelve — so let them train to become better at something they can do already.

Long series can happen by accident (I certainly didn’t think I’d write eight books about Jane Browne, but here we are — and more might follow at some point), but if you’re planning a project as a series, then there are a few precautions you can take.
First and foremost, don’t raise your stakes too quickly. You need to raise them from book to book, no question, but you don’t have to go from ‘local mystery’ to ‘the whole world is at stake’ between book one  and book two. It will still be a raised stake if you go from that local problem to one on a slightly bigger scale. In the first book, it was all about the village, now the nearest town is involved, too — or the neighbouring village is. In the third book, there’s another town or even a city involved and so on.
Like this, you will have a lot of books between those ‘local mystery’ and ‘the whole world is in danger’ situations. By the time it’s about the whole world, you characters should have a nice collection of useful skills, good friends, and enough renown to be taken seriously when they talk to the wizards’ council.
Alternately, of course, you can switch out main characters every now and then, let the children or younger siblings or apprentices of your first-generation heroes take over. Yet, most people want to end a series with the same main characters with whom it started, so either establish the changing heroes early on or find a very good reason for the exchange.

A series bible also comes in useful at this point. I would always suggest to keep a list of characters and their skills handy and to note all plot points which might be important later on.
It’s helpful if you can just look into your notes instead of having to re-read several books (or their manuscripts) to find out what happened in that cave four books back and how it influenced this character’s spell-work. It helps you keep your characters’ current skill levels consistent, too, which is important. Even if you don’t note that they seem to have gained or lost a skill without explanation, some readers will. Better avoid that from the beginning.
If you’re planning ahead, you can also sketch out how the skills of your main and recurring characters should evolve and use that when planning the next book. It’s often easier to shape a plot around skills you want for your characters to get than to later on deal with a strange, disjointed array of skills they’ve gotten from various plots over time.

With all of this said, power creep can be avoided through careful planning and through avoiding to make the climaxes about the same thing every time. Introduce allies and the occasional MacGuffin, if you must, to make the resolutions varied. Make sure to evolve your characters’ skills in a logical manner that makes sense to the readers. Avoid raising the stakes too quickly, so the characters don’t need to develop their skills extremely fast to manage the next climax. Keep a list of skills your characters already have, so you can see whether one of those could do for the climax of your next book already (with a bit of a development, perhaps). Don’t let the power rise too quickly for just one character, either — try to keep them all on a similar level or you’ll have a situation in which only one character matters any longer (the ‘Son Goku Problem’).