Sub-project and sub-tasks

Hi,
I have an idea to better categorize tasks. Sub-projects and sub-tasks.

1) Sub-project should act like "folder" inside a Project, in order to group a series of tasks all related to a specific topic (for example, "checkout bugs" or "Performance issues", etc.)
2) Sub-tasks are similar but with a minor impact. It happens that a task (e.g. registration process) could be splitted in 2-3 other sub-tasks like "registration form", "privacy agreement", "set main preferences" (it's just a fake example) that sholud be related each other. We should have a link in Main one to each of his children, and maybe, "solving" all sub-tasks results in solving that the main one.

What do you thing about it?