Something that might be cool would be a readily usable adaptation of the Pathfinder rules to fit Thedas. That is, the setting for Dragon Age. It's big and relatively detailed, and it would be better explained by playing through the numerous games.
As some basic initial tweaks:
Classes
Thedas is a fairly low-magic world. Martial classes are much more common, but alchemy plays a major part (especially in Inquisition). Some classes with spell-like abilities such as Rogues with magic talents can still take the abilities, but they function as Alchemist Extracts.
Martial Classes available without tinkering:
Fighter
Rogue
Monk
Barbarian
Swashbuckler
Brawler
Slayer
Cavalier
Classes requiring tinkering. Archetypes that lose spells are fine, but they can potentially be replaced with alchemy as well. I might get around to designing some archetypes especially for these.
Ranger
Paladin
Inquisitor
Bard
Warpriest
Hunter
Occultist
Banned Classes:
Gunslinger - No guns in Thedas
Magical Classes
I'm a little on the fence about what would be best here, so I'll give an explanation as to what I think is most sensible.
Spontaneous magic seems to be the best thematic choice, since preparation doesn't seem to be a thing in the games. So I'm going to say regardless of what you pick, your magic user is always spontaneous.
Witch/Shaman - Clear theme here: You have either a Demon or a Spirit that is living in your body with you, which grants you magical powers. Probably want some specialised patrons here.
Sorcerer/Wizard/Arcanist - All of these could work. Have the same spells as sorcerer and it's just class abilities that change.
Oracle/Cleric - Divine magic as such tends to be low key. The Maker doesn't seem to have an active hand in world events. Witch seems to be more thematic overall, so both of these are out.
Bloodrager - I hate bloodrager because it makes regular barbarians redundant. But the idea of a demon of wrath or something living in your body is perfect for the Reaver class.
Alchemist - Needs no change at all. Perhaps the most common of the magical classes.
Medium - How can you say no to a medium in this setting? Perfect.
Kineticist - Tough call here. The at will stuff seems in keeping with the nature of the game though, so I will probably tinker with it.
Mesmerist/Psychic/Spiritualist/Summoner - Maybe there's a place for these, but they seem a little off for the setting.
Druid - The Dalish look like they have some sort of druidic tradition, so I'm tempted to say these are cool. Shapeshifting has been a thing from the start, too.
Magus - Arcane warriors seem dead on here. But starting as one is apparently impossible, so I'm calling this out for now. Either accessible as a prestige class (with different spell progression) or you'll have to go for Eldritch Knight.
Basically to summarise that, all martials, Alchemist, Sorcerer, Medium, Witch, Druid. And any other class with an archetype that removes magic, or swap it with alchemy. All casters are spontaneous.
Races
Humans are naturally in and need no tweaking. Dwarves might need some minor adjustments since they're now especially magic resistant. Elves likewise need a bit of tinkering, since they're a little different now. But they should mostly be fine.
Qunari are roughly equivalent to half-orcs judging by the flavour. Doing a Mabari race could be cool, but definitely a challenge to get to work. Golems... might be awkward. Lost Spirits or Demons are another possibility, but again these are sort of awkward.
That's all of the races, I think. Call it Human, Dwarf, Elf, Half orc. And half elf, I guess.
Cool and different stuff
I think what I like best in Dragon Age is moving away from traditional Int/Wis stats. Keeping in that spirit, I think I'd suggest bringing in a new stat: Magic. For the sake of it, Intelligence is now called Cunning and Wisdom is now Willpower, but they otherwise function as normal.
All spellcasters use Magic as their casting attribute instead of whatever they would normally use. If your class has spell-like or supernatural abilities, they also always use Magic to determine all effects and uses instead of whatever they normally would. Spellcraft and Use Magical Device are based off Magic instead of Cunning and Charisma.
I'm of mixed opinion of how to deal with this for non-casters. Either they don't get it at all and Magic is a stat is possessed only by magic users (with ~3 extra BP to compensate for more stats) or it has some beneficial effect for everyone. The beneficial effect would likely be something along the lines of 'Adjust effectiveness of alchemical effect by 10% for each point of magic modifier'.
Special materials
The different materials have been a big part of the game since the start, and inclusion of some of these would be cool. Might take a little while for me to design these properly, but Grey Iron, Silverite, Red Steel, lyrium &c will probably come at some stage, with various interesting effects! For now, Silverite is approximately Adamantine and Red Steel is mithral. Dragonhide and dragonbone will be quite different to normal.
Magic items
Ha ha, you want magic items? Tooooo baaaaaaad!
Um, yes. Runes are how it does it, so runes it is. That would be how you can enchant arms and armour. Plonk a rune or two on them. Staffs are definite, major things (although their function is so different they'll need to be reclassified as weapons). Rings and amulets seem to be things as well, but the nature of them is such that combining the two into accessories seems like the thing to do. Potions are major things, and there's some limited evidence of wands. Scrolls and other wondrous items are apparently right out, but making mage 'armour' will be something that I'll probably push under craft magical arms and armour.
I'm very, very tempted to rework item crafting all over to make mundane the best way of doing things (which it clearly is in game).
More might come in a bit, but there are some bare bones for tweaks to get a more Thedas-y feel. Post below and let me know what you think!
As some basic initial tweaks:
Classes
Thedas is a fairly low-magic world. Martial classes are much more common, but alchemy plays a major part (especially in Inquisition). Some classes with spell-like abilities such as Rogues with magic talents can still take the abilities, but they function as Alchemist Extracts.
Martial Classes available without tinkering:
Fighter
Rogue
Monk
Barbarian
Swashbuckler
Brawler
Slayer
Cavalier
Classes requiring tinkering. Archetypes that lose spells are fine, but they can potentially be replaced with alchemy as well. I might get around to designing some archetypes especially for these.
Ranger
Paladin
Inquisitor
Bard
Warpriest
Hunter
Occultist
Banned Classes:
Gunslinger - No guns in Thedas
Magical Classes
I'm a little on the fence about what would be best here, so I'll give an explanation as to what I think is most sensible.
Spontaneous magic seems to be the best thematic choice, since preparation doesn't seem to be a thing in the games. So I'm going to say regardless of what you pick, your magic user is always spontaneous.
Witch/Shaman - Clear theme here: You have either a Demon or a Spirit that is living in your body with you, which grants you magical powers. Probably want some specialised patrons here.
Sorcerer/Wizard/Arcanist - All of these could work. Have the same spells as sorcerer and it's just class abilities that change.
Oracle/Cleric - Divine magic as such tends to be low key. The Maker doesn't seem to have an active hand in world events. Witch seems to be more thematic overall, so both of these are out.
Bloodrager - I hate bloodrager because it makes regular barbarians redundant. But the idea of a demon of wrath or something living in your body is perfect for the Reaver class.
Alchemist - Needs no change at all. Perhaps the most common of the magical classes.
Medium - How can you say no to a medium in this setting? Perfect.
Kineticist - Tough call here. The at will stuff seems in keeping with the nature of the game though, so I will probably tinker with it.
Mesmerist/Psychic/Spiritualist/Summoner - Maybe there's a place for these, but they seem a little off for the setting.
Druid - The Dalish look like they have some sort of druidic tradition, so I'm tempted to say these are cool. Shapeshifting has been a thing from the start, too.
Magus - Arcane warriors seem dead on here. But starting as one is apparently impossible, so I'm calling this out for now. Either accessible as a prestige class (with different spell progression) or you'll have to go for Eldritch Knight.
Basically to summarise that, all martials, Alchemist, Sorcerer, Medium, Witch, Druid. And any other class with an archetype that removes magic, or swap it with alchemy. All casters are spontaneous.
Races
Humans are naturally in and need no tweaking. Dwarves might need some minor adjustments since they're now especially magic resistant. Elves likewise need a bit of tinkering, since they're a little different now. But they should mostly be fine.
Qunari are roughly equivalent to half-orcs judging by the flavour. Doing a Mabari race could be cool, but definitely a challenge to get to work. Golems... might be awkward. Lost Spirits or Demons are another possibility, but again these are sort of awkward.
That's all of the races, I think. Call it Human, Dwarf, Elf, Half orc. And half elf, I guess.
Cool and different stuff
I think what I like best in Dragon Age is moving away from traditional Int/Wis stats. Keeping in that spirit, I think I'd suggest bringing in a new stat: Magic. For the sake of it, Intelligence is now called Cunning and Wisdom is now Willpower, but they otherwise function as normal.
All spellcasters use Magic as their casting attribute instead of whatever they would normally use. If your class has spell-like or supernatural abilities, they also always use Magic to determine all effects and uses instead of whatever they normally would. Spellcraft and Use Magical Device are based off Magic instead of Cunning and Charisma.
I'm of mixed opinion of how to deal with this for non-casters. Either they don't get it at all and Magic is a stat is possessed only by magic users (with ~3 extra BP to compensate for more stats) or it has some beneficial effect for everyone. The beneficial effect would likely be something along the lines of 'Adjust effectiveness of alchemical effect by 10% for each point of magic modifier'.
Special materials
The different materials have been a big part of the game since the start, and inclusion of some of these would be cool. Might take a little while for me to design these properly, but Grey Iron, Silverite, Red Steel, lyrium &c will probably come at some stage, with various interesting effects! For now, Silverite is approximately Adamantine and Red Steel is mithral. Dragonhide and dragonbone will be quite different to normal.
Magic items
Ha ha, you want magic items? Tooooo baaaaaaad!
Um, yes. Runes are how it does it, so runes it is. That would be how you can enchant arms and armour. Plonk a rune or two on them. Staffs are definite, major things (although their function is so different they'll need to be reclassified as weapons). Rings and amulets seem to be things as well, but the nature of them is such that combining the two into accessories seems like the thing to do. Potions are major things, and there's some limited evidence of wands. Scrolls and other wondrous items are apparently right out, but making mage 'armour' will be something that I'll probably push under craft magical arms and armour.
I'm very, very tempted to rework item crafting all over to make mundane the best way of doing things (which it clearly is in game).
More might come in a bit, but there are some bare bones for tweaks to get a more Thedas-y feel. Post below and let me know what you think!