Egon Bianchet wrote:
It could be simpler, end-user oriented: you would be able to define and choose colors schemes (just say five colours)
Just to add a further negative to this idea - most users have no design sense and no taste. Giving them control of layout and colour is mostly going to result in sites with bad designs and bad usability (lack of contrast between colours etc).
Yes, you can come up with a restricted set of things they can change, but you're not only making you theme creation job harder, you're also removing most of the reason for doing it in the first place.
Good designs are created by good designers. Great designs are also targeted to a specific purpose. Trying to create these über themes that do everything for everybody will force you to do lowest common denominator design, and at best you'll end up with capable mediocrity.