(I'm crossposting this from Story Games, because I figure people here might be interested. It's written with that audience in mind though - hopefully that won't be a problem)
A belated followup to this thread. You can find Sufficiently Advanced here.
Our group has a bit of a Planescape obsession, but D&D is an awful fit and I've yet to see a convincing argument for a system complementing it. Sufficiently Advanced has a few things in its favour:
1. The character's beliefs are mechanically significant.
2. The game assumes lots of different cultures which you'll be traveling to - and they're mechanically significant as well.
3. Everything is exceptionally lethal or can do horrible things to your characters - but you have a way out if you need it. It will cost you though.
4. Your character can be as powerful as you like. You can mix different powered characters together and it won't break the game. Weird characters that don't fit in the normal bounds work just fine (this is why we decided not to use TSoY - we couldn't work out how to allocate stats for, say, my shapeshifting wild mage created out of the chaos matter of Limbo idea, because you can't be generally powerful).
5. Isn't player-character centric like The Pool (what we would normally use for Planescape for lack of better options) - the multiverse can *actually* be hostile.
Convinced that this was a good idea, we started on our hack. The main thing that needed an overhaul was the Capabilities - in their original form they correspond most closely to D&D's core stats - that would have been a fairly straightforward conversion but it wouldn't have made much sense for the setting - magic just isn't classified that way. So instead we went for changing them to the different schools of magic. This involved quite a bit of butchery (for instance - conjuring/summoning now relates to all movement - if you're running away or, say, using a Haste spell (normally Alteration), you'd roll this capability). In actual play it's turned out to be more robust than expected - we haven't run into any situation that we couldn't catagorise under the slightly stretched definitions for each school.
Here are the tables we came up with (still probably a bit rough, but meh):
1. You get hurt. Often.
2. Normal person's defences.
3. Skilled at ducking/soaking up damage.
4. You've no need of armour - your skin serves just as well. (e.g. Shield, Armour - 1st-2nd level spells)
5. Arrows and lesser spells bounce off you. (e.g. Protection from Normal Missiles, Lesser Globe of Invulnerability - 3rd-5th level spells)
6. Magic nor weapons can touch you. (e.g. Anti-magic shell, Repulsion - 6th-8th level spells)
7. Pretty much impeneterable - those trying to to attack hurt themselves. (e.g. Prismatic Sphere - 9th level spells)
8. You can go one on one with dragons, point blank - and have them die from sheer exaustion. You protect your whole kingdom. (e.g. Lefeber's Weave Mythal - 10th level spells)
9. You can protect your whole Demiplane. Deities have trouble hurting you. (11th level spells)
10. You can protect anything, anywhere, from anyone. (12th level spells)
Alteration:
Changing things into other things.
1. Lacking basic ability to manually (i.e. nonmagically) craft/modify something.
2. Normal ability.
3. Masterwork craftsman
4. Change your mass, slightly modify appearance (e.g. Enlarge, Alter Self - 1st-2nd level spells)
5. Change into an animal, fundamentally alter a substance (e.g. Polymorph, Transmute Rock to Mud - 3rd-5th level spells)
6. Minor reality alterations, powerful transformations (e.g. Reverse Gravity, Polymorph Any Object - 6th-8th level spells)
7. Temporarily alter reality, readily alter any living or nonliving matter (e.g. Time Stop, Shape Change - 9th level spells)
8. Alter an object of any mass, or any creature (e.g. Proctiv's Move Mountain - level 10 spells)
9. Alter the fundamental laws of reality for a particular area (e.g. Proctiv's Breach Crystal Sphere - level 11 spells)
10. Alter the fundamental laws of reality on a permanent basis over any distance for anything (e.g. Karsus's Avatar - level 12 spells).
Conjuration/Summoning:
Deals with movement.
1. Clumsy, slow.
2. Normal abilities.
3. Champion Athlete.
4. Summon normal animals or an unseen servant. (e.g. Summon Mount, Summon Swarm, Unseen Servatn - 1st-2nd level spells)
5. Summon creatures of minor power, double your own speed (Monster Summoning I, Haste - 3rd-5th level spells)
6. Summon powerful creatures, bring items to you from over great distances (e.g. Invisible Stalker, Drawmij's Instant Summons - 6th-8th level spells)
7. Can draw powerful beings to them (e.g. Gate, Monster Summoning VII - level 9 spells)
8. Can summon objects and forces of enourmous power (e.g. Mavin's Create Volcano, Tolodine's Killing Wind - level 10 spells)
9. Can summon any being of lesser power than a deity (level 11 spells)
10. Can summon anything, over any boundary (level 12 spells)
Enchantment/Charm:
Measures the persuasiveness and charisma of the char. It is the ability to convince others, manipulate them through words, display capability of leadership, manifest eloquence. Art of social interactions. Mind altering and domination effects also fit.
1. Words are not your forte.
2. You are a social being.
3. Orator's eloquence.
4. Convince any humanoid that they're your friend, cause unreasonable fear. (e.g. Charm, Friends, Scare - 1st-2nd level spells)
5. Create emotional reactions in others, plant ideas in other's minds (e.g. Suggestion, Emotion - 3rd-5th level spells)
6. Compel someone to do whatever you wish, charm everyone around you. (e.g. Geas, Mass Charm - 6th-8th level spells)
7. You can raise an army for any cause. (9th level spells)
8. Your voice enthralls myriads.
9. Strangers kill themselves at your word.
10. You can talk water into travelling uphill and convince deities to give you their domains.
Illusion/Phantasm:
Lies, sleight of hand, and general trickery.
1. Hopeless at lying.
2. Normal ability.
3. Master conman or magician.
4. Create three dimensional images, turn invisible. (e.g. Change Self, Mirror Image, Invisibility - 1st-2nd level spells)
5. Create realistic illusionary scenes. (e.g. Hallucinatory Terrain, Vacancy - 3rd-5th level spells)
6. Powerful realistic illusions, permanent illusions (e.g. Programmed Illusion, Permanent Illusion - 6th-8th level spells)
7. Can create an illusion powerful enough to kill (e.g. Weird - level 9 spells)
8. Can create permanent illusions that cover vast spaces (e.g. the spell Leira used to hide the civilisation on Selune from Faerun - level 10 spells)
9. Can create illusions that are impenetrable to all but deities (level 11 spells)
10. Multiverse spanning illusions that can fool deities (level 12 spells)
Invocation/Evocation:
Offence.
1. You fail at beating things up.
2. You have normal ability.
3. You're a badass.
4. Unavoidable missiles, create and throw balls of fire (e.g. Magic Missile, Flaming Sphere - 1st-2nd level spells)
5. Firey explosions, hurl lightning bolts (e.g. Fireball, Lightning Bolt, Cloudkill - 3rd-5th level spells)
6. Deadly acidic fog, cloud of fire (e.g. Death Fog, Incendiary Cloud - 6th-8th level spells)
7. You can *fire meteors* at people (e.g. Meteor Swarm, Bigby's Crushing Hand - level 9 spells)
8. You can devastate kingdoms - rain fire from the sky for a week (level 10 spells)
9. You can devastate worlds (level 11 spells)
10. You can devastate Planes (level 12 spells)
Necromancy:
Matters of life, death, body and biology. Healing goes here.
1. You are alive and will die, probably soon.
2. First aid, average constitution.
3. You are an adept healer.
4. Heal wounds, minor attacks on the soul. (e.g. Cure Light Wounds, Larloch's Minor Drain - 1st-2nd level spells)
5. Animate corpses, return the dead to life, directly attack an enemy's life force. (e.g. Animate Dead, Raise Dead, Enervate - 3rd-5th level spells)
6. Destroy life force, regenerate lost limbs. (e.g. Finger of Death, Regenerate - 6th-8th level spells)
7. Steal life force, instantly kill all those immediately around you (e.g. Energy Drain, Wail of the Banshee - level 9 spells)
8. Raise armies of the dead instantly, kill all in a given area (level 10 spells).
9. You can kill nations with a thought, suck the life out of worlds.
10. You can raise dead gods.
Divination:
Knowledge, intelligence, problem-solving skills, logical thinking. Bullshit detector.
1. Clueless berk.
2. Average intelligence.
3. Genius-level intelligence.
4. Learn facts without being told, see through invisibility, detect motivation and intent. (e.g. Identify, Know Alignment - 1st-2nd level spells)
5. Scrying, talk to those in distant locations (e.g. Clairvoyance, Contact Other Plane - 3rd-5th level spells)
6. See through all lies, discover lost lore. (e.g. True Seeing, Legend Lore - 6th-8th level spells)
7. Forewarned of future danger. (e.g. Foresight - 9th level spells)
8. Understand stuff that would take several lifetimes. Your mind holds stuff that would break the minds of others.
9. Cosmic knowledge - you know how the Planes work. Stuff that cannot be comprehended with several lifetimes.
10. You know everything about everything… past and future.
The other thing that didn't work for us was how themes/capabilities interact at character creation - we wanted some firm limits on what you could put as your capabilities (outside of your civilisation) - and didn't like the hard upper limit of for your maximum capability. Fortunately, someone else had already worked this one out for us here. This ends up looking like this:
Import 5 gives 512 points (enough to put 8 in each category).
Import 7 gives 256 points.
Import 8 gives 128 points.
Import 9 gives 64 points.
Also, the character's race is a free profession in the same way as locality - automatic 5. This was to account for innate abilities that are far above the character's actual capability, but makes general good sense - you should be pretty good at being what you are.
That's basically it as we're currently using it, but we have some more thoughts being thrown around in email that haven't made it into play yet. One thing we've been arguing about a lot is the role of spells and items. 2/3rds of the group would like them to have mechanical effects - say, a Shield spell automatically negates a Magic Missile attack without rolling, or an Invisibility spell giving +10 bonus to the result of a relevant Illusion roll. The other player would prefer to not change this part of the system to keep it simple. No one's really willing to force their point of view at this point, so we're sticking with the status quo.
Something else I proposed was that the players of off-screen characters would be able to roll against their core values to have some minor influence in scenes that they're not in (and are relevant to that belief, obviously). Almost all of our games have been only with two people so far though.
We're also considering how to make it less necessary to hammer only one particular reserve pool during extended conflicts - at the moment (as we're reading the rules, anyway) there's a strong mechanical disincentive to change tactics during an extended conflict - if you go from trying to win someone over to trying to kill them, all your effort in getting their Charm reserve down is wasted. We don't have a satisfactory solution yet.
A brief sketch of actual play:
In the main game we're running of this at the moment, my character has (mostly through force of belief ('don't be evil: 2') and extremely lucky dice rolls) stood her ground against an Abyssal Lord who wished to dominate her mind and have her murder her friends so that she could be molded as his apprentice/queen (my character is an alu-fiend with a 3 in the Romance theme, if that helps put it in perspective).
Having failed utterly to even scratch her charm reserve, he decides that good is sexy anyway and proposes to her on the spot (the 'befriended by the enemy' complication in play). She decides to stay in his realm, given that this helps her quest for knowledge and she's afraid that if she doesn't he might kill her - but is rather lukewarm to the idea of becoming his queen. Using her Empathy theme, she's gained the trust of his undead blood-drinking sister and is on the road to making the 802nd layer of the Abyss a tolerable place.
Of course, who knows what will happen if the extended conflict with the Abyssal Lord trying to win her affection turns out poorly. His capabilities are in the demigod range, and one of his core values is 'Inflicting eternal torment on those that wrong him: 9'. Furthermore, my character's romance descriptor states that her romances tend not to work out too well. I anticipate a horrible future for my character.