Generally speaking...
Under
Warrior, I split Cavalier/Samurai and Fighter (Myrmidon)/Bushi into their own classes, combine Swashbuckler (Fighter) and Swashbuckler (Thief) with... fewer/less Thief skills, Swordsage (Kensai). Paladin and Ranger get more spellcasting access earlier; Paladins absorb Sohei, and Rangers get better Thief abilities instead of the favored enemy stuff.
Myrmidon and Swashbuckler keep the Fighter XP table. Cavalier and Swordsage use the Paladin/Ranger.
Under
Rogue, Thief picks up some of the Yakuza territory stuff. Assassin works as reprinted in
The Scarlet Brotherhood, but uses the Fighter XP table; they gain partial spellcasting at 4th level, learning Mage spells from the Illusion, Shadow, Dimension, and Necromancy schools, plus the Ninja spells from PHBR15. Bard gets less Mage spell access (Major Divination and Illusion, Minor in all other schools of philosophy) in exchange for Song Magic specialization and limited Priest spells; they can learn (as a Mage) All, Astral, Charm, Divination, Healing, Numbers, and Thought.
The Hunter class is more a less a combination of the Scout and Bounty Hunter Thief kits, nerfed skills but better combat.
Rogue classes are
hard; except for giving them access to different forms of spellcasting. They either feel
too niche or not differentiated enough from each other. I'd like to have a sapper/saboteur rogue, maybe, and and do something with archaeologist/investigator. Possibly Trader, which has precedent in Spelljammer and Dark Sun. What I
DO NOT WANT, explicitly, is a courtier or "social" rogue.
Mage classes are probably the easiest. Pick the most iconic specialist mages, trade in some of their school access for other benefits. Or try to replicate some of the later arcane classes. Krynnish moon-mages. I assume modern D&D automatic spell acquisition and use 5e prepared/known with
Spells & Magic spell points; different classes might use different casting systems from that book, but I'm only spitballing here.
For me?
- Elementalist/Wu Jen; all Elements plus Song, Alchemy and Geometry, awkward casting method; or Sha'ir, if I can find a better English-language synonym than bard.
- Mountebank/Beguiler; Illusion, Mentalism, Dimensionalism (pick one to specialize); minor Force, Shadow, Song, Wild Magic. Rogue Proficiencies, some Thief abilities.
- Some sort of sage magician-- maybe even "Wizard"-- that gets Abjuration, Divination, Conjuration, plus Alchemy, Artifice, and Geometry. Specialize in one. They can cast unprepared spells as "free magicks" in exchange for extra spell points plus "ritual" casting times.
- Stealify the Witch traditions from Timothy Brannan's OSR supplements.
- Not the biggest fan of Wild Magic, but sure, chuck it in-- Wild Magic specialist plus the least subtle schools.
Priests are also pretty easy. Kill the Cleric, give the Druid and the
Spells & Magic Crusader, Monk, and Shaman Major: Healing. Crusader doesn't have reduced spellcasting, Monks gets
some of the
Scarlet Brotherhood monk abilities, Shamans are less "primitive" and gain Major: Astral and Divination and some 1e Shukenja/3e Shaman bits. Four classes isn't bad, and gives us plenty of variety to differentiate different nonhumans.
Do we want more? Take the Athasian Elemental Cleric, add minor access to the Mage school of their element-- and you've got something between an Athasian Cleric and an L5R Shugenja.
Psychics are a pain-in-the-ass, but obligatory; psionics are
core rules in AD&D in the same fashion that
Oriental Adventures is a core rulebook. I will not be dissuaded from this.
But all of the 2e psionics systems are
ugly as sin, and the 3.5 psionics system doesn't
feel like AD&D. So what I'm going to do is nick
psychic magic from Paizo's
Occult Adventures, using the same spell points and a different spellcasting mechanic than any of the Mage or Priest classes. A lot of work I'm not that interested in doing right now, but basically cherry-picking the Mage and Priest spell lists to construct Psychic disciplines.
The Mystic is a psychic class; it's the
Scarlet Brotherhood Monk again, in full, replacing a lot of its class features with psychic spellcasting.
The Psychic is the "pure" psychic class.
Throw in the Soulknife because Soulknives rule.
But I've got more important things to work on.