Color By Numbers Series
The unending series and its problems
Vital though it is to have a way to keep the character from changing in a way that makes having more adventures either implausible or unsatisfying1 -- vital though it is to avoid escalating the series until it's silly2 -- the most vital thing for a series is to have new ideas.
This does not mean a new villain in a new setting.
It means a new plot.
Especially in the reveals. If the first two books have an Obviously Guilty Person who is not the villain, the third Obviously Guilty Person will fool no one. If a bit of procedure is explained for mildly contrived circumstances in the opening of the first three books, and proves relevant later, every reader will know it is Significant. But that is merely the aggravated case.
Tracking down the red dragon in the mountain lair and the green dragon in the forest lair and the white dragon in the snowy north lair only works if the different aspects alter the story. If the rocks and cliffs, the thickets of the forest, and the snow and ice do not change the effort of getting in, they are not actual changes. The dragons then have to differ in how to fight them, and how they fight, so as to require new plans.
In particular, you should make large alterations, early, in anything you do not want to keep forever.
This is because you are setting expectations for the series. The charm of a series, causing reader to pick up the book beyond the expectation that the writer is good, is to see variations within the pattern of the series.
If a wizard is sent out to discover what caused a magical problem, and for the first four books, she detects the wizard behind it, neutralizes the problem, and brings the wizard to justice, when the fifth book turned out to be that the migration of the fire lions stirring up the other magical beasts, or even a magical object that has started to act, you have a problem. (Throwing in someone profiting off it does not fix the problem.)
This is like throwing a haiku in a sonnet sequence. The world's most excellent haiku does not fit in a sonnet sequence. A mixed series of different lyric structure, or even a sequence of interchanging haiku and sonnets could work, but it has to be set up from the start.
For the same reason, it is far better to have, from the beginning, a mix-up of different problems in different stories, so that the wizard must detect the cause, deduce how to fix the problem, and determine whether there is a culprit, and what relationship if any the culprit has to the problem, before bringing any culprit to justice.
This is not to say that it is impossible to change and keep going. I have seen a series change like that. One where the character was assigned to a new task and immediately observed she had been promoted to tasks of greater responsibility. But this raises the specter of character change for the main character and of escalation, both of which threaten the series with inability to go on forever. Also, it can lose its charm for the reader.
So it can be pulled off once or maybe twice in an unending series.
Or, of course, it could remain a series of unmasking villainous wizards. That requires an endless stream of inventive new ways for wizards to cause trouble, and new and different ways for the protagonists to determine what they are doing, and stop them.
Within the parameters set by the series. Lord Darcy had a good long run in which you could reliably tell that, despite the presence of functional magic, that the murder would never turn out to be magically performed. The trick was novel ways to perform the murder, and novel ways to detect it, including by magic.
Repeating an obviously useful spell, say to detect whether the wizard is using fire magic, or whatever, is one thing. But the knowledge should sometimes be clear, sometimes misleading, sometimes absent. (And our intrepid wizard talks of how spells can be used to manipulate other things, leading to no traces where the eventual effect occurs.)
Likewise a trick that works one time should be repeated only rarely, and it's best if it works only with reworking, and that reworking is not merely doing the same thing only more. The characters have to twist it.
Or, of course, they could fail through overconfidence in it. Possibly even realizing what twist they should have used, after the fact, when the first use burned that possibility.
Even that, despite the series' parameters, needs to be rare if the series is to be kept fresh.



Yeah, Harry Potter book 4 broke the rule of "mixing up the thing that should be used rarely". In book 2, half the plot is brewing a poly juice potion to disguise the heroes as someone else. In book 4, the villain uses it without any of the difficulties the kids had. Also in book 4, a villain kills someone, transforms the corpse into a bone, and buries it. It feels so cheap and lame. I was unsatisfied with book 4 in ways I wasn't in any of the other books. I heard she got stuck on it and had to rewrite it, so that's probably why.