Those of you who use Delicious.com know what I'm talking about. They offer tag groups.
Todoist labels are great, but would be greater if individual labels could be assigned to a 'label group'. This would make is more hierarchical.
I do agree with Thomas. That feature I've been already thinking of would be useful, because, at least concearning me, I've got some projects where a need to group labels. Than developing the idea, I would be nice to search tasks by typin labels' group name.
Definitely a feature I would like to see to categorize label 'people' to label 'location' etc ...