I realize this has been discussed on this list rather recently (starting with Jon's CologneBlue question), and I realize some exploratory work has started (or at least was considered), but I'm submitting this anyway. I came up with the general idea first (really!), just haven't had time to write it down before.
Several people asked me to reply regarding the CologneBlue thread – consider this my response. :)
Comments (here or on the talk page) would be very welcome. I haven't gotten anyone to formally commit to mentoring this project yet, hopefully that can be sorted out on time.
https://www.mediawiki.org/wiki/User:Matma_Rex/Separating_skins_from_core_Med...
Project synopsis:
MediaWiki core includes four skins, and allows site administrators to create and install additional ones. However, the process is less than pleasant, due to several related problems (lack of documentation, more than one "correct" way to make a skin work, directory layout that makes packaging and (un)installation difficult, core skins and MediaWiki itself being interdependent, and possibly others).
I intend to solve at least two of the aforementioned issues by devising and documenting a saner directory layout for skins (and applying it to the four core ones) and then carefully disentangling them from MediaWiki code, removing cross-dependencies and making it possible for non-core skins to have the same level of control over all aspects of the look&feel as core ones currently have. This would make the lives of both skin creators and site administrators wishing to use a non-default skin a lot easier.
If everything goes well, the process would be culminated with moving the core skins out of core, to separate git repositories. This would require coordination with MediaWiki release managers (to have them shipped in the release tarballs the way certain extensions are shipped now) and Wikimedia Foundation Operations team members (to ensure the deployment of the new system on Wikimedia wikis goes smoothly), so it cannot be made a part of my core proposal.