Also take into account on the javascript redesign, javascript wiki-side
extensions.
[[MediaWiki:Common.js]] importScripts [[MediaWiki:Wikiminiatlas.js]],
[[MediaWiki:niceGalleries.js]] and [[MediaWiki:buttonForRFA.js]], which
then load [[MediaWiki:buttonForRFA/lang.js]]... plus the several Gadgets
the user may have enabled.
On Wikimedia Commons I load 38 scripts located at the MediaWiki
namespace (plus gen=js).
I'm pretty sure loading all of them when they aren't in the cache slows
it much more than the organization of the core mediawiki javascript.
Transcluding in the same request files would benefit a lot (either
automatically detecting calls to importScript or with a new syntax).
Finally, a dependence you may not have taken into account would be that
some CSS from the shared repository should be usable by host wikis when
viewing the pages.