"Weird Gods of Sword and Sorcery" or something along those lines, a big thick monster manual size listing of weird, swords & sorcery deities. It's the same idea of Judges Guild's excellent Unknown Gods, but taken to ridiculous extremes. Mostly a bunch of petty, niche godlings, the kinds that Conan or Elric would stumble across.
Although that's wasn't my original intent, a monster manual of the sort of bush league jerk-demigods that or Conan or Elric would end knocking over, isn't the worst resource for picaresque high-level play...
Anyways, so you have this books and there's a ton of gods in it. There's a god of swords, a god of axes, a god of siege warfare, a god of sharks, a god of earthquakes, a god of forest fires, a god of dancing girls, a god of slime monsters, a god of sea monsters, a god of mules, a god of arson, and so forth and so forth. And none of them have any big, campaign altering elements. And they're all nicely somewhere suitably between archetypal and weird.
When a new player asks if they can have a Cleric of the god of poison, or the god of apes, or the god of flowers, or some other random or way-out god, this book could have a fair chance of providing. Plus, for a DM being able to browse the index and think that "...maybe I want cultists of the god of Wrestling in my campaign..."
Another element could be associated minor artifacts for appropriate deities, with appropriate associated complications; another resource for emulating Elric-esque High Level Sword & Sorcery, "Our adventuring buddy is possessed by this ancient, holy headsmans' axe, and so far has proven to be nigh unstoppable; we can either kill him or kill the god of Decapitation..."