I would like to see an easy switch between hiding subtasks within their projects (current default behaviour for most calendars) and viewing them as stand-alones (as currently in the starred/today views), plus another option (perhaps, less important) to view them indented under their parent (project) tasks. This is particularly important in the All calendar, but would be useful in other calendars too.
Sometimes I want to get an overview of my tasks, for which existing view is fine, but sometimes I want to see the order of my tasks and priorities spread across all projects, which is impossible to do.
Ideally this would be an onscreen switch like the focus filter, rather than in settings, but even in settings would be useful.
This is the reason I mostly use the Toodledo website (which has this feature)rather than the 2Do app.