There are some key questions...
Should I go with 3.5 or 4th edition?
Regardless of edition, should I change the mechanics of powers/spells?
Fable 1 used mana. Spellpoint variant could do nicely.
Fable 2 incorporated "charge magic". Which sucked in alot of fights as you could never charge up to full.
I thought of incorporating charge magic, but that would slowdown the game. As it is I don't like innitative checks and turnbased, rather just say what you want to do but don't abuse the leniancy! Charge magic would become very rules heavy in designating the number of rounds you spend charging and glorify innitative checks and turnbased, ugh.
I've thought of maybe changing the weapons to weapon groups too. If I want to go with Fable's policy that an iron broadsword is alot different then a steel broadsword..
The plothook writes itself. The PCs are "heroes". Which is I guess Fable's word for people born with extradionary power. Because it's not about good or evil, but organizing these super powered individuals for the purpose of guild or other interests.. It only makes sense, seeing as PCs are going to be so much more powerful then their normal humanoid kin.
If 3.5, I considered maybe only allowing classes with inborn magic who learn innately and don't study. Like sorcerers, psionic classes, supernatural ones from Tome of Battle or Tome of Magic...
4th edition is good with ritual magic. But only npcs use rituals. Like those assclown brothers always getting me involved in a new quest because they can't putdown The Necronomicon.
Alignment is a big issue too. Regardless of if I use 3.5 or 4th edition. Spells, abilities, feats, etc. Are going to be effected by alignment and grant bonuses of some kind. Essentialy in Fable 2 corrupt/pure=law and chaos. And ofcourse there's good and evil. Ironicaly it just takes chaos to grow horns as my character is "chaotic neutral" and he's got bigass horns. These sortsof physical characteristics are minor but can effect how people perceive them and help them out. Horns are good for natural attacks, halos give you a reliable source of light in darkness. (Even supernatural darkness)
The only problem with 3.5 is the gross imbalance of spells. Fullcasters will always be better then noncasters. Even though for such a game where basicaly all the heroes exhibit magic, noncasters would be discouraged. Even the "Rock Lees" like Hammer exhibit supernatural durability and strength. Probably a psychic warrior or something.. So I guess that wouldn't really be an issue. Not to meation I could incorporate those "magic veins" that seem to permeate strongcasters if they abuse magic too much. And generaly civilizations don't act kindly to "will users". It scares most people when you use it in public, can get you arrested, and even in the more savage lands they find it to be in bad taste and petious.
4th edition might also be better cause of the power sources.
Strength=Martial/Primal
Skill=Marial/Psionic (using that one dude's homebrew classes)
Will=Divine/Arcane
I could limit it down to you could only multiclass within your "hero type". Albeit there isn't much point as multiclassing is unheard of in 4th edition except to dip into a prestige class... In 3.5 such limitations would be suicide on the strength based. When in Fable, having alot of HP is astoundingly important as there isn't a healing spell or buffs.
It might be cheesy and a bit too videogamish. But I could also sell food as a means of healing one's self or granting temporary stat bonuses. For example, an apple might heal.. 1d2hp. And raise your lawful alignment by 2 points. A turkey leg would grant you a temporary +2 to your strength bonus for 1 hour, 2 chaos alignmnent points, and after that hour you must make a fortitude save or become fatigue. (Turkey makes you sleepy, lol!)
I think essentialy all PCs and sentinent npcs would be human. But just take on different apperances and mannerisms based on where you come from, and fluff regarding magic and alignment. For example, someone from bloodstone, which is a heavily populated area filled with lots of shops might have shorter people to accomodate for the crowds. (Haflings or Gnomes?) A place like wraithmarsh where it's total anarchy and primitive tribes fight tooth and nail for survival cause it's all swamp and deadly monsters might look more ragged and muscular (orcs and shifters). Another edge for 4th edition because races are more balanced and diverse. There's no penalty for roleplaying from stat penalties.
But I'd like to hear your thoughts. The only reason I lean 3.5 is so often people are disenfranchised from 4th edition. I think it's nostalogia myself. As 3rd isn't all that great.. I could easily houserule save-or-dies, stat damage, and unfair distribution of attributes and HP by forcing dice rolls. So I don't see why everyone hates on 4th.
I thought about if I did go fourth making it more customizable with no classes. But they build their characters from the groundup and select powers from different classes. However, I'm not quite sure how balanced that would be as there's a reason specific classes gain specific powers... It might be kind of game breaking to have someone who could be a controler and a striker at the same time!
Minor action is something, move action is a little better, standard action is best. You could even go nuts and let people use multiple actions or all 3.
Of course if you're going to keep balance then you need to give a boost to non-casters too.
I would probably keep the benefits relatively small or this will quickly get obscene.
+1 to hit, a small damage boost(+2 ish) an extra square or two of area for a burst or blast(or maybe it can be moved differently), that sort of thing.
Weapon wise--the quality of weapon material corresponds easily to different +'s of magic items, iron=normal, steel = +1, etc. Most augments have an equivalent too (flame=fire,etc) and are a nice way to let players "make" their own items.
For the rest of the mechanics I wouldn't worry about it, I don't think they add enough to be worth the hassle (and the casting is already questionable), you can fluff it and say that people are eating during short rests (and that's why they get to spend surges) if you want. Alternatively you could make them potions, an apple potion gives 10 HP for example and costs a surge....
The reason to do it is for the flavour of the fable world and not the mechanics, so I wouldn't worry about how to do them 100% but more of how you'll represent things fluff wise.