> So any task with `Parent-Id` will be a sub-task. If User owns the tasks he can see the subtasks tree also. but if you own the sub task alone, for you it is like a normal task with an out of scope parent.

That should work.

> Recursive tasks works exactly like recursive calendar events.

I'm not sure what a recursive calendar event is. Are you reading "recurring" instead of recursive?

> I have not thought about Sub-Projects, any suggestions?

The same `Parent-Id` solution as subtasks should also work here.