But even freeing the assets eventually would be really nice.

There's plenty of creative effort involved around extending and improving existing content from old games, or making something new loosely or not-so-loosely based on the originals, but this often involves creating derivative works of the original proprietary assets which means these mods are illegal.

In some cases you can work around these limitations with code. E.g. in the doom community you find that making custom palettes is very popular nowadays, as the original makes some bad tradeoffs and many nice color schemes are out of question without a custom palette. But to completely overhaul the palette, you must re-do the original assets or create some sort of a mapping that source ports would understand.

Problem with this model is fact that in modern games a lot of code is scipts that are part of assets.