If nearly everything moved out of this repo into taps, what would stay in? Either actually in this repo, or in a homebrew-core tap.
Probably git. Other things?
gettext, pkg-config, build tools.
IMO we need to keep control over a significant number of popular libraries, otherwise the fragmentation will result in things not working with each other; glib, qt, boost, etc.
Things for which brew uses <thing> is huge.
brew uses <thing>
Things for which we want to enforce the keg-only property.
Agreed with what Jack says. The most popular packages for developers (e.g. autoconf, automake, cmake, qt, boost), console utilites (e.g. ack, colordiff, fdupes, gist, hub, phantomjs, imagemagick) and for local development environments (e.g. mysql, postgresql, redis, elasticsearch, rbenv, ruby-build). Those are mostly based off my preferences.
The main things I'd like to see go from core: Qt-based GUI applications, the entire GTK stack except maybe glib, obscure development libraries and development libraries with no or very few brew uses.