When I read fluff I thought you meant background materials at first (Like this: *spoiler Cataclysm DDA*
https://github.com/CleverRaven/Cataclysm-DDA/blob/master/data/LAB_NOTES *End of spoiler*. Stories about where the monsters came from or something. But you mean additional game content.
I don't really have anything to contribute apart from listing what kind of fluff Crawl Stone Soup has. It is all content you can do when you are already capable of winning the game. It just provides more score, or more stuff to do with your character while still being challenged. Not sure if you are looking for that, or just for more paths that make your character stronger to take on the endgame.
In Crawl stone soup you need to perform 3 steps to win. You need to collect 3 of the possible 15 runes, go down to the lowest basic dungeon level and collect the orb of zot in the realm of zot, and then go back up and escape.
Optional parts:
- During the normal dungeon run the various optional branches such as the ice caves. They provide only loot at expense of some additional danger.
- Collecting all 15 runes. There are a few very hard ones in the different Hells, and Pandemonium (Randomly generated super hells).
- Going down a ziggurath to level 27. (Each level of a zig is progressively harder than the first, and while almost any build can win, very few builds can reliably get to a zig level 27).
- After getting the pandemonium runes, any pan level.
The only things these provide is more exp, more items, and in case of the 15 runes some score. But they are optional.