we need sub folders
this option is already available on Todoist. To create a sub-project, right-click on its name, choose "Edit project" and then use the arrows on the right side of the input box to indent/un-indent it to create a sub-project on up to 3 indent levels. Alternatively, you can use the keyboard shortcuts: Ctrl+RightArrow and Ctrl+LeftArrow.
Same applies to task - you can create sub-tasks on 4 indent levels.