Somewhere at the end of a universe, there is a bar.
In this bar is a pretty brunette staring intently at a laptop, humming idly to herself and occasionally scribbling on an attached tablet.
"I'm very grateful you didn't decide to write over my reality, too!"
"Whatever you want, basically? I was planning to not have it be attached to any specific world, and instead have it be perched dramatically in some incredibly cool place. Do you want a door that can borrow other doors in different worlds, so you don't have to have a library? Do you want a lot of rooms in case you have a lot of guests? Do you want your house to change its layout to suit your ever-shifting needs? If you had the perfect impossible dream house that you kept until the end of time while you go running off on multiversal adventures, what would you like it to have?"
He beams widely. "Well, I like the idea of being able to use doors to get to it, and if I'm gonna be going to other universes it might be useful for it to always match. Other than that, though—I'll give you free reign."
"Hmm, all right. So, house that can comfortably pretend to be a house in the worlds that you go to. That'll mean the layout and style will change, though I think there should perhaps be secret sections where the layout at least doesn't change. Like, uh, I bet you'd appreciate some kind of trophy room, where you can collect things from your travels. Probably also an armory that'll stay the same, so you know where all of your things are. Both should probably expand as necessary, and maybe change to match the style of what the house is being, but I don't want you to not know where your stuff is. Especially when it's important stuff. Do you want like, impressive and intimidating, or homey and inviting? Probably it'll vary a bit depending on what world you're in, but I'm wondering which direction to take it."
Giggle. "All right." She taps her fingers against the arm of the chair and looks thoughtful.
"... All right," she repeats, clearly coming to some kind of decision. She holds out a hand, and a key attached to a key ring drops into her open palm. She offers it to James. "Here is the key to your house. Use it on any door with a keyhole, and the door will open to your house. This isn't as good of an emergency exit as the library card, so I'll just leave it at 'you can retrieve it from any place that could conceivably hide a set of keys in it.' So, pocket, sleeve, bags, impressively poofy hairstyles... you get the idea. The layout and style will try to roughly line up with what would make sense on the other side of the door, but might fudge a bit to make you look more impressive. In the house there will be, hm.... large paintings or pictures of your armory and your trophy hall, in whatever style best fits the world you last came from. And then you, uh, knock thrice on a painting and then hop through to get to your stuff. All sounding good so far?"
"Good! Oh, and it'll try to have hot water, a large comfy bed, and a closet with clothes that match the style of the place you came from. Aaaaand probably it should come with some way to immortalize people you care about, because you might collect those, and I'm hardly going to demand that you be immortal and watch people you love die. Uh - something that can scale comfortably, so you can dump it in a world and then ignore it if you want, and not have to worry about doing the responsible thing?"
"Well, yes. But usually the characters of the authors in my social group think it's worth it, and that space to live and resources to take care of people are things that can be acquired. Recall that we've been to the Moon and that there is a very big universe out there. Acquiring more things and space isn't as hard as it is on Tyria. Whereas dead people are often lost forever, and even if they're not, the time they spent being dead, away from everyone they love, is wasted, and all of the pain of everyone that outlived them can be averted. Granted there would probably be massive societal shifts and a lot of freaking out by everyone, but. I vaguely expect someone you run into is going to want to save everyone, it's kind of our genre of thing."
"Huh. Well, from what I heard the Underworld is pretty dull, so that makes sense anyway."
"... It depends on the author, there are other afterlives that can catch people. Some of them are significantly nicer than the Underworld, some of them are significantly... less nice." She swallows. She is not going to enjoy the next part. "But sometimes they also just, um. Stop. And then that's it, they're gone."