For DMs and players both...

I'm planning my next campaign (entirely homebrew plot and setting, which is where I shine) to be a level 3-20 high-magic post-apocalyptic fantasy game. It will be a classic-ish campaign, not focused necessarily on heavy survival elements, although that may factor in early game. (Methods around survival elements will be accounted for in a way that doesn't become obnoxious later game.)

I'm curious as to what sort of ideas folks might have for such a setting, for plot hooks they'd be interested in if they were playing in such a game, and any other such nuggets. I'm not asking anyone to build my game for me, just for some inspiration, as there's little I've found aside from Dark Sun that seems to align with what I've got in mind.

Things I've already got rattling around my head: more wild or unpredictable magic, pockets of unstable gravity, spaces where the planes have converged, thinning veils, and a reason why high level magic users and/or the gods haven't just magic'd/wished the world back to being fixed.

Thank you all! :) 

If you haven't taken a look at it yet, I highly recommend Van Richten's Guide to Ravenloft, and I would suggest looking at the domains of Darkon and Hazlan for inspiration for an apocalypse.

There's a setting that I loved but never got a chance to actually play from back in 3.5 called DragonMech that was post-apocalyptic steampunk high fantasy. There was an obvious bias toward the steampunk aspect focusing on the giant mecha the setting takes it's name after, but it includes plenty of magic as well (notably the elven answer to mechs is "animated mechs" which are basically huge, piloted golems). The premise is that the surface of the moon of Highpoint (which is the name of the world and the large continent the setting takes place on) started breaking up and falling on the world below as massive, civilization wrecking meteor storms. Also, the moon is inhabited by a bunch of nasty creatures that came down with the "lunar rain" and continue the said wrecking of civilization. Surface dwellers flee into the underground, often coming to violent odds with the people already living there, and eventually an old dwarf appears from the deep underdark and teaches the folks to build giant steam and clockwork powered mechs to fight the lunar menace with. The elves eventually mimic those mechs with magical versions once they prove successful and even some necromancers have similar ideas (including a mad archlich who builds a walking necropolis with the stated intention of destroying the moon itself). Several new nations form based around fleets of mechs, some so big to be actual walking city-fortresses, as they carve out safe zones by reclaiming the surface from lunar monsters. There are still various nomadic barbarian tribes who have adapted, and other fringe groups as well. Adventure hooks include intrigue, sabotage, and even potential outright warfare between the new nations and factions, exploring unreclaimed territory including strange ruins from the moon, and dealing with crazy moon cultists. 

The key factor that, in my opinion, makes the setting so rich narratively is, rather than the obvious hook of the steampunk mechs, how everything revolves around the apocalyptic situation of the Lunar Rain in some way. Even after the meteor showers subsided the world was drastically changed and people had to adapt, which caused them to rebuild society to look significantly different than it was before. Along that line, I would suggest you focus your world building on what the apocalypse that your game takes place after actually was and what it did to mess up and otherwise change the world. How do people live differently now, and what lingering effects are there? 

Off the wall suggestion:

The apocalypse made humans extinct. They no longer exist, but their buildings and artefacts still remain.

Elven archaeologists are confused (look at how they built this, it's almost like they couldn't see in the dark, but that's ridiculous!).

"Mid-High Magic & Post-apocalyptic,” hmm, lemme think about it…. Well, here are a few I can think of off the top of my head:

• The Forgotten Realms
• Greyhawk
• Blackmoor
• Mystara
• Ebberon
• Dark Sun
• Dragonlance

Ohh, and don’t forget:

• Middle Earth
• Shannara
• The Dragon Nibmus Universe
• Mithgar (The Iron Tower)
• The Lords of Dûs setting

Ooooh, Shannara's a really good one!

A good place to start for inspiration could be Dark Sun, an older DND product. Other than that, look to stuff like Conan and Mad Max and combine the features you like.

In Dark Sun's universe the gods just left, so you could be inspired from that in some form or another.

I guess it really depends on what apocalypse you had, and what that has meant to the world. Are the players among very few people left alive? Are they all warforged, trying to uncover the mysterious circumstances of the end of the world? did the world end forever ago, and it's now rebuilding?

Inspiration, not worldbuilding, hm?  There is one post-apocalypse setting everybody tends to forget that had some pretty wild ideas worth cribbing.  To summarize, may I present a quote:

"Long ago in a distant land, I, Aku, the shape shifting master of darkness, unleashed an unspeakable evil! But a foolish samurai warrior wielding a magic sword stepped forth to oppose me. Before the final blow was struck, I tore open a portal in time, and flung him into the future where my evil is law! Now the fool seeks to return to the past and undo the future that is Akuuuu!"