As I said several times it is possible, a bit complex and not very ergonomic. There are surely some infrastructure evolutions to consider for the management of some todo to become a real task management. This is not the priority of the development team at this stage.
It all depends on the level of complexity you have to manage (number of projects, connection between them, collaboration, etc).
I add that if you already use a GTD software (e.g. everdo) that suits you or a custom task software for GTD (todoist or other), you can now use the "external links". You copy the link to a note in the description inside your GTD application and it allows you to "jump" to Joplin with the right note selected when you need it.