I'm sure everyone's had the problem of making a save years ago and not being able to pinpoint what custom bricks it actually uses, making you say that it requires stuff that it really doesn't. What I suggest is a decent way to detect non standard bricks, according to their names in a .bls file, and then spit out what brickpacks they actually are from a database... This could probably work as a website, and could also probably work as some kind of program or even an add-on, though a website would be preferable.
Perhaps this could function as an extension to a save file upload site, too, so that people know which addons a save file requires, but that's another suggestion.