I've done this, not really for D&D but earlier, when creating a set of attribute scores for a computer/console RPG design. My own set had (mostly independently-arrived-at) analogues to what are Strength, Dexterity, Constitution, and Intelligence in D&D (different names for some), but I ended up creating Awareness and Presence.
Awareness represents the acuity of your physical senses and the degree to which you are "in tune" with your surroundings. An exceptionally high Awareness means that not only are your physical senses very sharp, but you also have this sort of "sixth sense," as your awareness of your surroundings begins to extend into the otherworldly, the mystical, the spiritual... Empathy. Intuition remains a part of Awareness, as it does not represent ideas arrived at through the process of logic, but ideas which just suddenly you (quite possibly from what you are able to pick up on, either consciously or unconsciously).
Presence takes on most of what Charisma does and becomes more of a measure of inner strength. Personal magnetism, persuasiveness, the ability to lead -- or more specifically, the ability to make people want to follow you. Willpower definitely belongs as an aspect of this attribute. It's all about how people perceive you, and that projection of inner strength to the outside world -- confidence and self-assuredness -- is what lies behind many of these aspects that tend to attract people to you in droves. Physical attractiveness plays a role, but would be a fairly minor aspect. And I think that "Presence" is a better description of the D&D attribute "Charisma" even without the added Willpower aspect.
So this cleans up Wisdom and make it make a little more sense. Awareness is still a critical attribute for Rogues, Rangers, and others that make use of skills like spot or listen. It still makes sense as the primary attribute for Clerics and Druids and perhaps is a bit more focused on relevant aspects than when Willpower was part of the package. Awareness should perhaps have some effect on social interaction skills, perhaps depending on the circumstances, due to possibly being more perceptive of and in tune with another's emotional/mental state and a genuinely empathic understanding of his or her situation.
Presence makes Charisma a more useful/valuable stat than it was previously, as it would obviously replace the Wisdom bonus to Will saves. And I think the re-worked "Presence" fits as the primary attribute governing Sorcerers' and Bards' spells a little better as well.
Hmm, I think I might implement these changes in the next campaign that I run (which has been in the works for some time).
