Hi,
When the rig or the scene contains a lot of elements, nudging elements backwards and forwards on the Z-axis (Alt + Up Arrow & Alt + Down Arrow) can have a visual impact.
That’s why, I think that it would be great if there was the possibility to animate the display order without changing z-position values.
It would be cool, if the animatable value of the display order position would be an offset added to the element’s “real” display order position (which is based on the node view hierarchy).
On the other hand,
if there was a way to “mute” node attributes like described here, everything could be done via a peg.
No need of an extra module. 
Some more thoughts on this feature here.
I think that it would be useful to have two different modules allowing to animate this attribute.
A Peg
A Display Order Offset node, especially for this attribute.
This would be useful for nodes that may not be animated individually.
A such module would spare the user from having to expression-block all other animatable attributes of a peg.
Just watched the Movies Insider’s “How ‘The Simpsons’ Animation Evolved Over 30 Years” documentary.
Among many other interesting facts it mentions that digital painting and compositing allowed the makers to create crowd scenes that wouldn’t have been possible to make with traditional cells.
https://www.youtube.com/watch?v=BEieYcoUqLA&t=461s
I think that an “Animatable Display Order Offset” feature would have a comparable importance for crowd scenes made in cut-out animation in Harmony.
I still would love to have this feature.
I think that it would be a cleaner solution compared to counter-animating scale to compensate z-position changes in order to maintain the visual size.