There's something to this three-tiered approach that I've been taking with Saga of the Splintered Realm...
To review, I'm designing three books at once: a book of core rules (inspired by the 1981 BX books), a book with a campaign setting (inspired by the 1984 Greyhawk boxed set) and a megadungeon environment (inspired by what I know of Castle Greyhawk).
There is a synergy that arises out of these things coming together. When I go through and stat out a monster (for instance), I'm thinking of several things at once:
1. How can I retain the 'spirit' of the monster as presented in those original rules? How can I bring its abilities in line with the unified mechanics that the core system uses?
2. How does this creature impact/interact with/belong in the campaign setting? Where is its place in the world? How where might it impact the progress of mankind and his ilk in any meaningful way?
3. How does this creature end up in a huge megadungeon? How does it interact with other creatures in that environment?
Because I'm not solving the second and third questions 'hypothetically' but actually, I'm ending up with some different ideas and views than if I had just done one book at a time. I don't see any huge revelations here (nothing like 'all people are actually reincarnated dragons of the lost age!'), but mostly firming up of ideas and logical connections arising between various game elements. Here are some decisions that I'm pretty much locked in on right now - although we are still early in the process:
The game will cap (mortal limits) at level 14. I like this a great deal. 14 was the default cap from 1981; it's a logical consequence of events in the game world (variations on the Splintered Realm have always had some default ceiling for human advancement as a result of the Great Reckoning); this makes the demi-human limits far more palatable. I'm okay with my dwarf only being able to get to level 12 if the human fighter can only get to 14 anyway. That's only 2 more levels. It is a huge problem when the assumption is that humans could eventually get to 36, and there your poor halfling was locked in at level 8 for eternity. This also keeps the world from spiraling into exceptionally high magic. Magic Users can't go around dropping alter reality and wish spells all willy nilly. This also makes +3 items the top end of what's out there for mortals. +4 is reserved for relics, and +5 for very powerful artifacts of the distant past. It keeps power escalation in check a bit better all the way around - both in the game world and in the game.
Roll under will resolve all checks. You want to roll high for attacks and damage; you want to roll low for almost everything else. Instead of modifiers, almost everything will be listed as a rating, with your target as that number or lower on a d20. A thief 8 has sneak 12; he successfully sneaks on a roll of 12 or under on d20. Magic items won't replace this number (for the most part); they will grant a bonus to it; elfin boots grant +8 to sneak; an elfin cloak grants either +8, or with the boots grants +12. Now, that level 8 thief has a sneak of 24 with the cloak and boots on; you are not going to see him unless conditions (like an enemy's sense) modify this below 20, or he rolls a natural 20 on the check. Almost every type of roll is being converted to a check: initiative, surprise, morale, turn undead, reactions, rolling for treasure... once you internalize this rule, you never have to look up a rule again. A modifier has to modify the target and not your roll in order to remain intuitive. If I get +4 to my strength check to knock down a door (it's a pretty weak door), I'm adding +4 to my strength rating (the target) for this particular action.
AC escalates. I considered making attack rolls a check as well, but the application of the modifiers becomes clunky, or has to break the rule for all other checks. To whit: my foe is heavily armored. with an AC of 1. I am a level 1 fighter (+1 to hit) with strength 17 (+2 to hit) for a total modifier of +3. I modify the AC of my foe to 4 for this attack. Since modifiers always change the target (and not your roll), you constantly have to re-figure the target for each new foe. I know that you could subtract the bonus from your roll every time, but that feels counter intuitive. 'I have a +5 attack modifier, so I subtract 5 from my attack roll every time'... it just doesn't feel right to me, and also breaks the fundamental mechanics of the rules I'm trying to emulate.
One saving throw will rule them all. Your save will be based on your class and level, modified by your prime requisite modifier. A cleric 1 with Wisdom 18 has a save of 7. He uses his wisdom to save against any effect. His wisdom allows him to minimize the impact of the snake's venom, cast his eyes away at the perfect moment to avoid meeting the gaze of the medusa, and withstand the mind-controlling powers of the harpies.