One often overlooked NLE is the sequence editor in Blender. It isn't nearly as feature rich as kdenlive, but my son uses it a lot and brought to my attention a particular feature which is rather nice and I wold like to suggest for a possible feature in kdenlive.
The feature is called meta-strips, which would be better called meta-clips in kdenlive lingo. The feature provides for the creating of internal hierarchy within a project, which can be used to significantly un-clutter a complex timeline and simplify editing in many cases. It works by selecting any number of clips and transitions, then selecting the menu item to turn those into a meta-clip. The selected items are then hierarchically turned into a single clip on a single track, much as if you had imported an external kdenlive project as a clip. Furthermore, it is possible to enter and exit the meta-clip and edit its contents if necessary. The meta-clip can also be "flattened" back into its individual components if desired. Effects and transitions can be applied to a meta-clip as if it were a normal clip.
The group clips functionality in kdenlive provides some grouping ability, but it doesn't visually un-clutter the timeline, nor add true hierarchical structure within the project. Meta-clips would sort of be an evolution of the group clips feature.
Just an idea...