As Brad has noted, S2 is 'imminent' in an other-people-get-to-contribute way. Personally, I have bold visions for the launch of this big new feature, and I want to see it launch with loads of styles, all of the relevant documentation and everything ready for mass user-adoption.
As part of this, I do think that all of the current system styles should be revamped and made in S2, and if possible these new (identical to the user) versions should replace the current system styles transparently.
This wouldn't have been an issue if it hadn't been for the mass-adoption of Style Overrides by free and paid users alike. S2 is radically different to the current style system, and style overrides in their current form will have no place in the new system.
I am keen that we do 'automagically' convert all of the existing free accounts over to S2 styles so that the new funky customisation features become instantly available to them with no kludgy switchover procedures. In this case, then, the question remains... what do we do with the style overrides the free users are using currently?
An obvious solution is to convert them into S2. However, since I like everyone else haven't seen anything about S2 since the original taster docs Brad published months ago, I can't speak on the feasibility of that. It also creates an issue when the users later want to modify their overrides and find the interface has changed drastically.
A final, related point is existing styles. Ideas that have been explored include an S2 style which renders current-system styles (since S2 as it will work on LJ will be compiled into perl, this is doable), a script to do a one-time conversion of them all, or just running the two systems in parallel. Whichever of these is chosen, there does need to be a migration path from the old styles to S2 other than remaking the styles from scratch. This is complecated by the fact that S2 styles are layered, and thus work a lot differently to the current system which only has three conceptual 'layers' - style, colour theme, overrides.
This is basically a call for discussion on the subject. I'm interested in what others think and can come up with on the subject.