Hi Rasa,
The Tasks sidebar can quickly become pretty long and would be easier to manage if tasks can be grouped and the groups twirled open/closed. Subtasks could be displayed as children of their parent task, with each parent task having a foldout arrow to expand/collapse the subitems.
For drag-and-drop, I’d suggest that if one task is dragged onto another that it becomes a child (subtask) of that task. And similarly a subtask could dragged out to separate it or parent it to a different task.
I know that may be a lot to ask for. For the time being, I can use filtering to view tasks by the project I am working on.
Thank you!