We begin in the The Plane of Towers. There are other humans and demihumans there; feel free to roll for what kind of murderhobo you find if the NPC has been stuck in the Plane for a long time.
PCs can find ways to escape and get to Vornheim or Ur Hadad, or Stalag, a Deep Gnome city's peak beneath the Universities of the Cthonic Codex. All of which I imagine existing in the same world.
Things that I want to inspire this campaign:
Dreams in the Lich House's Taenarum
DitLH's Black City project as well
So from these two, whatever city the PCs arrive in, they'll find or hear of some megadungeon just outside the city. If they want, they can travel there easily enough. Elsewise they can pointcrawl the city. Backswords and Bucklers may give me some ideas for city roguishness.
In terms of gonzo: Kalak-Nur is pretty inspiring to me.
I think Dreams in the Lich House really inspired me to play this using 5e Dungeons and Dragons. I think there will be a few minor modifications to test out in the Plane of Towers (characters run or have access to a small pool of characters, weapons and armor degrade) Definitely want XP to come from more than just combat, as well, to incentivize the players to hand me conflict kind of in the vein of this post, though that probably works just as well with combat-only DnD. I dunno, perhaps for now I won't mess with the XP system. Other than awarding XP for things like 'Achieving Important Goals' and 'Outwitting Enemies Such That Fights Aren't Needed.'
I recall someone running Vornheim and the players remembered Sharn, from the Eberron setting in DnD, but they couldn't actually recall that the city's name was Sharn; they were calling the city of Sharn 'the Eberron setting.' I can't recall if this was good or bad. On the one hand, Sharn is kind of the megacity of Eberron, so I can see people remembering the setting as this unique city. On the other hand, it'd be nice if people remember the difference between The City and The Setting, especially if they leave the city.
I want to make sure that each city has its own personality, on top of whoever the PCs are talking to / destroying. Vornheim is Cold. It was extruded slowly from the frozen Urth, and it will linger for a long time. It was made by ancient Men in ways we cannot easily replicate, but can repair. Ur Hadad is Mad. It is jovial as a rictus grin, its hallways and passages carved well before mankind, for reasons we cannot know. It is populated by jovial killers and stifled bureaucrats who think laws are the best way to play practical jokes.
Stalag would be my own making and has the most development to do. The deep gnomes are a contemplative, practical lot. They negotiate with everyone and act as middlemen. It is slow there, plodding, and quietly besieged. Drow and duergar maneuver around it, wanting it for themselves. There are lots of illusions, lots of the environment trying to twist around on the PCs, which would be damn hard to model. Definitely going to give the PCs some indications that 'this will lead you out of here this way' and let them decide how to leave.
Tangentally, Things missing from the Plane of TowersAnt Tower as given to me by this lovely post on Giant Ants.
Crystal Tower filled with crystal golems (using the Animated Armor from the 5e monster manual). A level 4 magus who created them. His murderhobo captives who are turned into living-heads-in-jars which are in turn used to power the golems. Inside each one you can see the face of someone. They seem to be screaming.
Needless to say, the PCs could soon have a murderhobo goon squad to go upstairs and wreck that wizard. Then, the problem is, the PCs have a murderhobo goon squad...
A besieged tower. This would probably let me use some of the mass combat rules from Red and Pleasant Land. I have to poke in there and see how those jive with 5e, and how to make horses/mounts super important for traversing a battle. I believe Zack already had that in there.
Regardless, inside there are nobles who have never left their tower and who typically dine on blood and peasants. Outside are a bunch of rabid humans who are there to knock over the tower. Many of them are foaming at the mouth and all are crazy evil.