The ability to expand sublime user setting in the .sublime-build, like the limited build variable expansion.

For example:-
In  makefile.sublime-build I could do the following set up
"cmd": ["make","${settings:buildtype}"],
"file_regex": "^(..[^:]*):([0-9]+):?([0-9]+)?:? (.*)$",
"working_dir": "${project_path:${folder:${file_path}}}",
"selector": "source.makefile", 
So as a user I could define a user setting "buildtype:"release" which I could change to "buildtype:"debug" to quickly change between build types, this would also allow me to easily create new ones, like "master", by simple editing my my build system (which is different to make) and changing the user settings, without having to create variants or edit the .sublime-build file etc.  Would be most useful for such things as multi-platform builds, as I could also define multiple user settings, for example settings for a  buildtype and buildplatform in so doing create multiple platform builds with "cmd": ["make","${settings:buildtype}_${settings:buildplatform}"]. I think this would be a simple but powerful addition to the sublime-build implementation.