I'd like to see the ability to use the "Mark as owned" and "Link executable" buttons for games that are already in our library. For instance, I might have a game on PS4 that shows in my library, but I also have the PC version and it's not automatically recognized, it would be nice to still import that. This would also prove as an effective way to handle modloaders, I think. Having my steam version of Sonic Mania and ManiaModManager both selectable under Sonic Mania, for instance, would be pretty nice.

For Mod loaders, you can add additional executables to your Steam game. Go to your games page in Galaxy, click the more button next to play, then hit configure. There, you can enable custom executables/arguments for your game, without needing to use your other platform slot as a placeholder.