I’ve been working on a setting based on a small island nation called Brynth which consists of a dozen or so cities. Long ago the cities were overrun by monsters, evil magic was used to create mazes within city streets, turning them into massive dungeon like labyrinths so that these days civilization has been pushed out into the surrounding forests, mountains and swamps (Think David Bowies Labyrinth but where every city is like that). There are no real towns or hubs of humanity. (Initially I wanted a survival/horror feel to the game, but it’s hard to create that without the world feeling empty/unfinished)
It’s still in the early stages and I’ve only played a couple of sessions in an attempt to help flesh things out a bit. I really want to give each labyrinth city it’s own unique feel.
Anyway, I just wanted to share this. Any ideas, suggestions or feedback is welcome.
PS: I’ve also got an idea for a larger time travel themed campaign using this setting where the players have to jump back in time to before the cities were all taken over etc… but I want to at least get one time period fleshed out first.
Sounds cool.
As far as advice goes, I'd suggest letting your players create pretty much just as much stuff as you do. Not only does that lesson your burden after you create the framework, but it also allows for much more fun (in my experience) at the table.
For example, if a PC is an elf, let them make up all the fluff/back story for elves in this world. Obviously you have to still create the major antagonists, and their minions, and their reasoning to do this to the cities. Maybe it was actually for the best, but no one knows that yet...But I digress...Let your players come up with as much as possible. How are the people pushed out of urban life even surviving? Are they captured for slave labor (or worse) sometimes? Again, your PC's should take an active role in determining a lot of that stuff.
That's something I do with my homebrew world, and it's been working really well over the years.
For labrynthine dungeon design (and other stuff), check out http://donjon.bin.sh/ It's a cool website with lots of tools and random generators. I'd actually call that site one of the best, if not the best, homebrew DM's tools.
Also, look not only to the movie Labrynth, but also movies like Dark City (the city reforms itself when people sleep, and no one remembers), Cube (a sci-fi horror where people are trapped in a shifting rubicks cube maze full of traps), The Stand...Even stuff like The Walking Dead, and Attack on Titan (all three deal with humans losing their cities/civilization, and nearing extinction). Maybe. I dunno exactly what flavor you want.
As far as the time travel thing goes, look to non-D&D inspiration as well. That idea reminds me of the SNES/DS JRPG Chronotrigger. It's on most top 10 (video game) RPG's of all time lists.
You don't need a lot details about the world to make time travel work, actually. If you start in the present, the past is somewhat easy to build in your world. It's better, lol. More technology, more trade, more happy people, more petty disputes that would be laughable to the present folks' situation...
Thanks for the response,
You're right about the wide influence, it's not just the Bowie movie, walking dead, dark souls 2, (I'm going to throw Game of thrones in as well, doesn't everybody want their games to feel like GoTs?) 
I also wanted the players to help build the world, but most (all but 1) haven't played since early high school, they're digging it, but getting them to roleplay is still a challenge, they tend to meta game a lot.
Although that said, pushing them to help build the world would be a good way to give them some ownership of the world and encourage roleplay.
After the first session I tried doing an email to get them to maybe write a little response (I spent some time doing pbem games, and find it's a good way to flesh things out between sessions. I didn't get much response so I dropped it. I think at this point it's still more of just a social thing to get together and hang out. Which is fine.
I'll try to post more about the world as we play in case peeps are interested.
Cheers