Hi Cristobal,
That's an interesting request! As a heads up, our developers receive an awful lot of feature requests, and have to prioritize them based on how useful the feature is, how many people would benefit from it, how long it would take to implement, etc. The more information you can provide about your request, the better. Specifically, it would be great if you could tell us a bit more about the specific business process you see this helping with, why the feature would be helpful, and why it's difficult to work without it. It seems like using the existing document relationship labels would would accomplish pretty much the same thing, so the more information you can provide, the better. Thanks!