So I few hours ago, to fight off insomnia, I posted this stream of consciousness dungeon and in room 5 I put your mom.
Which at the time was just the sort of the first thing that came to mind but now after sleeping, showering, and microwaving-some-chicken-and-noodles on it I am thinking this is a pretty good idea.
Ok see it's like this:
In the beginning there were modules.
Modules were adventures you could run with pretty much any group and they were, y'know, modular. They could be moved around. They were impersonal--they were full of treasure and obstacles to getting that treasure and anybody could give it a shot. Upshot here is: your mom was not in that dungeon.
Then things changed--TSR (for one) started doing this adventure path thing where the PCs were specific people with specific adventures and specific moms and often adventures wherein your mom is relevant.
That is: metaplot. The module was no longer modular, it was a link in a story.
Nowadays, at least here in the gaming avant garde where all us bloggers live, you have these two factions:
A-planned-story-(or-at-least-planned-theme)-heavy relationship-map-involving you-know-this-guy-and-hate-that-guy New Schoolers, and
B-throw 'em naked into the sandbox with the wolves and hope maybe one of them survives into the end-game Old Schoolers
Now it should go without saying that PCs in faction A often have moms. PCs in faction B sometimes have moms and sometimes don't.
In published adventures it is hard to deal with your mom: the appeal of such products is that they provide everything right there in the box. And so if they are going to put your mom in there than they almost have to put you in there to (i.e. give you a pre-gen) or else how do we know your mom is alive or what terms you're on or whether you even have a mom or were grown in a lab or whether you've already met her or... In other words, giving a character a relationship to the setting in a published product is hard because the designer doesn't know anything about your character or his/her previous adventures. Or at least it's hard to put your mom in there the way Old Schoolers like their moms: with no strings or plot rails attached.
Similarly, same problem comes up if you're building a sandbox for your own personal use: sandboxes are designed to be impersonal--that's their appeal as a challenge. (The Rat Temple will not be easier just because you are 3rd level when you walked into it.) If you put a character's mom secretly in a very specific basement in Slopwankia there's no guarantee the player will ever go there---because it's a sandbox. If you put the character's mom openly in Slopwankia then you are either saying "only go here if you want to see your mom"--which is the least interesting way to meet your mom.
Point is: unexpected visits by mom are interesting and funny. And they can provide the basis for hilarious role-playing for ages to come. However, it is hard to introduce moms into sandboxes or modules without the threat of also introducing pre-planned plots or arcs.
And I think my sleep-deprived dungeon actually has a pretty good solution: in this room is a PC's mom. The GM must figure out how she got there before the game starts and must establish a list of PCs whose mom it could be. Then roll randomly once the PCs show up, then play it out wherever it goes.
Anyway the whole point is, just because it's a sandbox doesn't mean you can't seed background stuff about themselves for the PCs to stumble into around it. And even if you're writing an Old School module it doesn't mean you can't describe certain useful lacunae and instruct the DM to fill them in with relationships appropriate to the situation in their own group. It won't be completely play-out-of-the-box and will require your DM do a little thinking--but then every module does anyway so why not?
I would like to see this sort of hybrid approach in adventures: don't tell the GM exactly what's there, but don't just be vague or "leave it open" either--describe to the potential GM what kind of thing needs to be there in order to make the scenario work and then describe what characteristics the missing piece must have. "The burgher of this town is a man with a vendetta against a PC but who doesn't know an important thing about the situation that inspired the vendetta". Then let the GM find a solution that fits his or her group.
People do this automatically a lot (everybody who playtested Vornheim hacked it a little to fit into their campaign) but it'd be nice if game designers started acknowledging this always happens and actually using the disjunction between the modules assumptions and the group's reality as a feature rather than a bug. Instead of going "If you don't have a good reason for the PCs to be here use this default one" take advantage of the fact that you don't know who is going to be in that dungeon and who their mom is, and make that part of what makes it fun by giving the DM a little design problem to solve.