The virtual clip creation via library, which was just implemented, is an interesting feature which is probably better than the approach other applications use for importing assets from a project to another. I guess that more sequences on the same project is anyway something that everybody needs in an editing system. The first program which used this approach was AVID. At today every professional system has adopted this method.
I use sequences all the time in editing, this feature in Kdenlive would be a game changer. Being able to import mlt and Kdenlive files as a virtual clip is essentially what sequences is, except you have to open their project to edit their contents.
I guess it becomes a bit more complex the moment you think about how Kdenlive would handle the project settings, as each sequence could have settings of their own.