Nate Hack

 Here's the basic idea for Nate Hack:

Base rules are AD&D

Ability scores, classes, advancement, spells, and experience gain are AD&D, except classes go up to 20

Skills, arts, and focuses are from Worlds Without Number, except I have a wider skill list based on D&D 5e. 

Encumbrance uses the readied slots system from WWN (equipped items use space).

Each class has access to a wider pool of arts (for instance, druids get beastmaster and elementalist in addition to shapechanger arts). I also wrote a pool of 40 combat arts for fighters, thieves, and monks, with powers like Quickstep, Mighty Shot, and Palm of Unfathomable Justice.

WWN spells are up-leveled to account for their greater power (for instance, a 2nd-level WWN spell is a 3rd-level spell)

Combat is mostly based on WWN. So there's group initiative, unusual WWN actions like screen and swarm attack, and players get their combat bonus to attacks. However, there's no shock, and we're using AD&D weapons and resolution mechanics -- so you get your AD&D Dex bonus to AC and your AD&D Str bonus to attacks.

The main wrinkle to combat is the Injury, Death, and Dying rules. Instead of dying at 0HP you choose between system strain and a temporary injury, and when you gain the same temporary injury twice it becomes permanent. 

The result is a tricky, tactical game with low lethality but high challenge, and the players have wide latitude to set the pace. I can introduce high challenge encounters and tricky traps and the players can play around with it without worrying about getting instagibbed, but if they take a lot of risks and play poorly, they incur a lot of system strain and injuries, and eventually might not be viable as a character anymore.

Getting a first level character up and running is tricky because of the large number of powers, but we're mostly experienced players who really like spending a lot of time thinking about this stuff, so it's been okay so far.

Characters are complex and have a wide choice of powers and focuses, so there are a lot of surprises during play as to what a character might be able to do, but for the most part they're limited by the basic ideas of AD&D -- invisibility mostly comes around at level 3, AOE and flight comes around at level 5, and so on. But the WWN spells, arts, and focuses give some interesting wildcards to keep the game from feeling too stale.

We've played maybe six sessions so far. They're in CRACKED MOON TEMPLE, the second dungeon in the game so far -- it's a temple to the elf moon goddess ZYSS, who governs illusion, insanity, dreams, poison, and transformation. She lives on the moon in a lunar palace, attended by lunar elves and 888 enslaved mortals. Her temple has been invaded by a black wyrm, and the players have been tasked with killing it, to open up the trade routes through the KELDWOOD that the wyrm has blocked. They've discovered a temple in disarray, where the elves have devolved into bestial insanity, and under the sanctuary of Zyss, found a secret passage to the secret temple of IXIL, Zyss's persona as the DARKMOON, where dark sisters and beast-knights wield swords of ice-cold steel and protect the eye-less, insane elf novitiates.