Content packs sound like a step in the right direction, however you still need to add a whole content pack to your project or create a pack for each individual asset.
When importing assets could UE4 not create a reference to the original and work from that?
When it comes time to compile and build your project, only the referenced assets used would be ‘hard copied’ to the project.
Putting something into practice is naturally a lot harder than it is to request, but I really think would improve the UE4 workflow considerably.