It would be helpful if the DefaultSettings could be overridden by environment variables. This would allow users to set the defaults for all wiki's on a system, but would also let easy configuration within a docker container or vagrant, etc. And would make MediaWiki closer to becoming a 12-factor app.
You would still be able to override the defaults with LocalSettings.php. However, a LocalSettings.php would technically not be required at all.
The order would be something like this:
DefaultSettings.php -> Environment Variables (if set) -> LocalSettings.php
The environment variables should take on the same pattern as the variable names for example:
$wgStyleSheetPath would become MEDIAWIKI_STYLE_SHEET_PATH or if we want to get the wg then it would be WG_STYLE_SHEET_PATH, but I think MEDIAWIKI_ is a better prefix.