TYPO3 Core

Currently you can just enable or disable concatenation and optionally exclude certain files. All files that end up in the concatenation will be added to one file.
It would be great to be able to specify some "concatenation-group"-parameter. All files belonging to one group will be concatenated separately. All files without that parameter are the main group.

This way you are able to skip merging files that you rarely use on your site (maybe just for a forum). But once you're below that section of the site you could separately load the additional (for example) forum-css-file with all individual CSS-files belonging to that group merged in.
At the moment if you add CSS-files for a certain subtree of your site only, that will create one large, separate concatenated file.