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Another angle to take with this in a series is to have s standalone novel in the same world with different characters. It's especially good for scifi/fantasy worlds to allow you to show off your world in a fresh way. This introduces new characters and different story lines, but is set in the same setting. It's a whole new opportunity for readers to become introduced to your universe without having to read the entire rest of the series first. In this scenario, the newly introduced characters can become weaved into the main story line, or main-line characters can be added to the new story line. Because the new characters aren't simple afterthoughts, they are given the chance to be fully fleshed characters without detracting from the original story. This has been done in any number of long-standing fiction series, like the Honorverse or Liaden books (the Liaden series book Fledgling is a particularly good example).

• Another angle to take with this in a series is to have s standalone novel in the same world with different characters. It's especially good for scifi/fantasy worlds to allow you to show off your world in a fresh way. This introduces new characters and different story lines, but is set in the same setting. It's a whole new opportunity for readers to become introduced to your universe without having to read the entire rest of the series first. In this scenario, the newly introduced characters can become weaved into the main story line, or main-line characters can be added to the new story line. Because the new characters aren't simple afterthoughts, they are given the chance to be fully fleshed characters without detracting from the original story. This has been done in any number of long-standing fiction series, like the Honorverse or Liaden books (the Liaden series book Fledgling is a particularly good example).
• If your series is highly successful, you may find yourself with lots of little snippets you loved but that didn't make it into the main line. A collection of short stories set in the world lets you get this content out without messing up the main story. And if you happen to write a short story introducing a new character, then who will question it? Once the character is established, they can be introduced to the story line. Maybe your short story collection has several short stories or a novella with the new character. Then you introduce the character into the main-line as a minor character with a steadily increasing and recurring role.

A series is a weave:

If your story is still in the outline stage, I think you're all right. If you have a plan, then as you go along in the story, you can allude to a person who will pop up later. Reference them and their relevant background in snippets as you go through the stories. So there is a chapter in each book referring to Bob in New Hope city. Maybe the characters even visit Bob. Perhaps Bob was married to one of the MC's sisters, or was a mentor. So when Bob shows up in book 3 (or Bob's son, if you want) Bob is already part of the weave of the story. The readers who may like your story can see that Bob was intended to be a pert of the story from the beginning. They won't feel you're doing a last minute addition because you alluded to Bob in the previous books.

There can be a logic to it even in the middle of the series. So maybe Bob is murdered, and Bob's son is the character you're introducing. Bob's son contacts the MC's and gets help avenging his father's death. The readers get to love Bob's son while tying Bob's son to the story line. Then if Bob's son does something the MC's wouldn't have because they are linked to a certain course of action, it can make sense. You get to explore parts of your world that the MCs simply never would have.

If you haven't published yet, then everything in the story arc is open to change. Nothing is canon until it's published. The allusions don't even have to be significant. Just enough that it's clear Bob was in your mind from the beginning.

A series is a weave:

If your story is still in the outline stage, I think you're all right. If you have a plan, then as you go along in the story, you can allude to a person who will pop up later. Reference them and their relevant background in snippets as you go through the stories. So there is a chapter in each book referring to Bob in New Hope city. Maybe the characters even visit Bob. Perhaps Bob was married to one of the MC's sisters, or was a mentor. So when Bob shows up in book 3 (or Bob's son, if you want) Bob is already part of the weave of the story. The readers who may like your story can see that Bob was intended to be a pert of the story from the beginning. They won't feel you're doing a last minute addition because you alluded to Bob in the previous books.

There can be a logic to it even in the middle of the series. So maybe Bob is murdered, and Bob's son is the character you're introducing. Bob's son contacts the MC's and gets help avenging his father's death. The readers get to love Bob's son while tying Bob's son to the story line. Then if Bob's son does something the MC's wouldn't have because they are linked to a certain course of action, it can make sense. You get to explore parts of your world that the MCs simply never would have.

If you haven't published yet, then everything in the story arc is open to change. Nothing is canon until it's published. The allusions don't even have to be significant. Just enough that it's clear Bob was in your mind from the beginning.

Another angle to take with this in a series is to have s standalone novel in the same world with different characters. It's especially good for scifi/fantasy worlds to allow you to show off your world in a fresh way. This introduces new characters and different story lines, but is set in the same setting. It's a whole new opportunity for readers to become introduced to your universe without having to read the entire rest of the series first. In this scenario, the newly introduced characters can become weaved into the main story line, or main-line characters can be added to the new story line. Because the new characters aren't simple afterthoughts, they are given the chance to be fully fleshed characters without detracting from the original story. This has been done in any number of long-standing fiction series, like the Honorverse or Liaden books (the Liaden series book Fledgling is a particularly good example).

A series is a weave:

If your story is still in the outline stage, I think you're all right. If you have a plan, then as you go along in the story, you can allude to a person who will pop up later. Reference them and their relevant background in snippets as you go through the stories. So there is a chapter in each book referring to Bob in New Hope city. Maybe the characters even visit Bob. Perhaps Bob was married to one of the MC's sisters, or was a mentor. So when Bob shows up in book 3 (or Bob's son, if you want) Bob is already part of the weave of the story. The readers who may like your story can see that Bob was intended to be a pert of the story from the beginning. They won't feel you're doing a last minute addition because you alluded to Bob in the previous books.

There can be a logic to it even in the middle of the series. So maybe Bob is murdered, and Bob's son is the character you're introducing. Bob's son contacts the MC's and gets help avenging his father's death. The readers get to love Bob's sone while tying Bob's son to the story line. Then if Bob's son does something the MC's wouldn't have because they are linked to a certain course of action, it can make sense. You get to explore parts of your world that the MC's simply never would have.

If you haven't published yet, then everything in the story arc is open to change. Nothing is canon until it's published. The allusions don't even have to be significant. Just enough that it's clear Bob was in your mind from the beginning.

A series is a weave:

If your story is still in the outline stage, I think you're all right. If you have a plan, then as you go along in the story, you can allude to a person who will pop up later. Reference them and their relevant background in snippets as you go through the stories. So there is a chapter in each book referring to Bob in New Hope city. Maybe the characters even visit Bob. Perhaps Bob was married to one of the MC's sisters, or was a mentor. So when Bob shows up in book 3 (or Bob's son, if you want) Bob is already part of the weave of the story. The readers who may like your story can see that Bob was intended to be a pert of the story from the beginning. They won't feel you're doing a last minute addition because you alluded to Bob in the previous books.

There can be a logic to it even in the middle of the series. So maybe Bob is murdered, and Bob's son is the character you're introducing. Bob's son contacts the MC's and gets help avenging his father's death. The readers get to love Bob's son while tying Bob's son to the story line. Then if Bob's son does something the MC's wouldn't have because they are linked to a certain course of action, it can make sense. You get to explore parts of your world that the MCs simply never would have.

If you haven't published yet, then everything in the story arc is open to change. Nothing is canon until it's published. The allusions don't even have to be significant. Just enough that it's clear Bob was in your mind from the beginning.