Something I would really love to see at GOG in the 'additional content' for games are some of the various tools and utilities that have been created.
For example, I remember Descent I/II had a fantastic fan-made level editor; I spent many hours creating maps for it. There's no reason that wouldn't be as fun today as it was 15 years ago.
And on that point, many older games had multitudes of user-created levels, skins, etc.
I'm definitely not asking for comprehensiveness here; but some of these classic games have extremely high-quality fan-made tools that would be worth giving the same second life as the game, and for anything multiplayer, including a consistent map pack for all users does nothing but enhance the fun.
All that said, I'm not hopeful; I imagine there's all sorts of legal issues with stuff like this. But it's a feature I'd love to see nonetheless.

i like this idea
i'm not that into the soundtracks and backgrounds--content for the game, official or unofficial, would really get me excited though

these things are typically listed in the mod section of the game's forums.

True but it'd be nice to get the latest version of say FSOpen when you buy Freespace 2 although I do grant that doing that for every game that had mod tools or notable mods would consume a fair chunk of storage and bandwidth
Maybe a html file that links you to the pages for the more popular mods?

My convoluted thought was to have a link per game via the game shelf page, to a forum thread. Said thread, being the existing mod thread, would likely be updated with the news on all releases of extra tools.
Of course, I know, it will be posted elsewhere per forum category, but it would allow us as the community to highlight all the stuff we use. Obviously(?? - hate using this word on forums ;p), we'd want to link to the site for the mod/tool, not the actual file to keep things upto date, and prod a few people into reading more about it than just downloading everything and not knowing why it didn't work immediately.