Count me as yet another fan of E6. I've been working on my own variation and would appreciate some critique and feedback of the following proposal:
While limiting characters to six total character levels and hit dice, I'm planning to allow characters to "rebuild" character levels in a process akin to the rebuilding discussed in PHBII:
For the expenditure of 5,000 XP, a character may exchange one existing class level for any other class level for which the character meets the prerequisites. A class level that forms an essential prerequisite for another class level cannot be rebuilt. The character must first rebuild an existing class level before any of that class level’s prerequisites may be rebuilt. If a class level that forms the prerequisite for an existing feat is rebuilt, that feat is lost and will have to be repurchased if the character ever again meets the qualifications. If a class level that forms the prerequisite for an existing maneuver or spell slot is lost, the maneuver or spell slot is likewise lost. Known spells for memorized casters (e.g., wizards) are not lost but may not be cast if the character lacks the necessary spell slots.
Because in many cases a prestige class may provide its own prerequisites, this technique may permit a character to rebuild one or more of the underlying class levels with prestige levels. This is possible even in the case of prestige classes granting caster level increases: even if all of the underlying caster levels are rebuilt, the prestige class levels “remember” their original derivation and are not affected by that rebuilding. For example, if a prestige class had a prerequisite that a character be able to cast 3rd level arcane spells and added a caster level at each prestige level, the character could, with 25,000 XP, replace all five of the original caster levels with prestige levels, which would still count (for spell purposes) as caster levels of the original class.
In all cases of rebuilding, a character’s BAB, saving rolls, skill points, class- and cross-class skills, power points, powers, class abilities, etc., are recalculated based upon the character’s new combination of class levels.