Nested clips
The ability to add nested clips would drastically improve the editing experience specially when doing compositing.
Here is a simple example of how blender does it:
A more complex way of doing it in Premiere, where it adds an extra timeline. Which could be thought of at a later moment:
https://www.youtube.com/watch?v=A8Aw53JBLZY
@massimostella has some more info about this in the editing manual