I don't really have time to participate in this in any but the most casual of ways. But from my observations of that last BC thread...
the very FIRST thing that MUST be done is get everyone in the core group agreed on what the taken can do, what the template does, and what the taken were like before the template. (And yes, I like the template idea.)
For example, the reasons why the later Taken weren't so powerful... was that because Lady couldn't do the job right, or because the original Taken were some of the most powerful Dark Archmages of all time, and the new ones were just any old 10th level+ spellcaster laying around?
Why were the Taken so indestructable? Was it because the had regeneration? Howler seemed covered with contingencies (frozen in Ice didn't stop him). Or maybe being Taken is just an odd form of Undead?
Once the nature of the condition of Taken is separated out from the spellcasting abilities, its time to correllate them to the abilities of Goblin, One-Eye, Silent, and Blade. Goblin/One-eye were illusionists, with a bit of enchantment. That's clear. But by the same token, those magics seemed to be part of all lower-order magics in the world. So maybe illusion is not a school, but merely a scattering of spells in other schools.
Blade had enough magics for a cantrip, One-eye had enough to cast 5-6th level illusion 8 times per day and live for 200 years, and the Taken had enough to do anything except for physical detonate their surroundings. Fireballs were about the most destructive they could get in a physical sense.
The magic-using classes also seemed fairly hard to progress on. One-eye and Goblin almost never seemed to level as spellcasters, yet they did lots of XP-earning stuff. (Research and item creation feats don't necessarily indicate leveling as a spellcaster.) Given Glen Cook's notion that magic makes you insane (which he said in an interview somewhere), maybe their are points in the Mage class where the character has to decide if he wants to proceed- and pay the price.
Once the magic system & classes have been resolved, it is now possible to figure out all else. But the differences in the magic seem central to the feel of the BC universe- the danger to individual heroes, the gritty violence- these are all house rules to the combat system which follow once you know how tough the toughest people are supposed to be...