I like my players to have a firm idea of their characters' backstories, and I like the idea of being able to draw on these (NPCs from the past showing up unexpectedly, etc.). However, in our next campaign the PCs will be exploring uncharted seas and previously undiscovered islands far, far away from home.
How can I weave in elements of the PCs' backstories in these far-flung and exotic locations where no one they know has ever ventured?
Best Answer
Use aspects that relate to the backstory rather that direct references to the backstory
I find that backstories are generally driven by events, personal trauma, people and objects. What you need is to push some aspect of these to the place they're visiting that each character can relate to or can use to resolve some aspect of their back story that moves/develops their character on.
Personal Trauma/Events
This can be a tricky one, and it may go over the characters head if it's too subtle or a car-crash trauma if it's too obvious; the way I've often used this is to use mirroring, presenting the character with a situation that mirrors some traumatic aspect of their history so they can relate and grow from it. Here you are fitting in a part of their history in an abstract but relatable way and that helps the character grow.
For example; when Bob the Wizard was but a young apprentice their master was kidnapped and ransomed by a Half-Orc Bandit, Bob didn't manage to get the money together to pay the ransom and never saw their master again.
Now there's two ways to do this; 1) The master turns up, but this doesn't help resolve Bob's childhood trauma, or 2) Bob meets a young boy/maid/apprentice/soldier whose master/father figure/etc has been kidnapped and a mini-plot around it helps Bob resolve some of his mental issues about losing his master by helping someone else do the same.
People and objects
This is harder; you either need to drop in an NPC (A younger sibling decides to stowaway on the ship with the Character to try and emulate them) but this is heavy handed and as you've said - only so many NPCs can pop up in exotic location - or some aspect or aid/history that the NPC needs sorting out can be resolved whilst in the exotic location - a cure for a long time disease, finding out about a rare book that the NPC would like; the important part is the NPC doesn't need to be there themselves, but some aspect of what either reminds the Character about them or something that they need/want can be bound into it and turned into a mini-plot; the book can be hard to find or bargain for, the disease requires a plant that grows inside a volcano.
Similarly with objects. If Amy the Shieldmaiden is carrying the sacred shield of her order, she could find a lost piece of info about her order, find a way to improve her shield with some kind of new material or spell.
TLDR: People are usually used to resolve or advance backstory, but aspects and ideas that relate abstractly or partially to their backstory can be used to push and develop backstory as well.