The manual says this for Zscale: "is an float value – the scale in z axis of the selected sprite.
I'm not exactly sure what that means or why you would use it instead of just making the Depth value larger. They both seem to do the same thing to me.
That being said, I think a good feature to put in the future would be the ability to bring an object further away or closer in a layer without making it any bigger. For instance, a lot of games automatically make the playable character "4 Depth". What if you would like the character to maintain that "4 Depth", but bring them 8 pixels closer to the front of the screen? Right now you can bring the character those 8 pixels closer, but only by selecting "12 Depth", and this makes the character 12 pixels deep instead of 4 deep.
Here is a great example of why you would want to do this, using the game Blaster Master:
Ideally, you could keep the power bar at 8 units deep, but bring it in front of the landscape and enemies that would otherwise get in the way of you seeing it. Right now, it's possible to bring it to the front, but this is how it ends up looking and I don't think it's very appealing:
EDITED TO ADD:
Here's an example of how Double Dragon II would benefit from ZOffset as well.
No matter what I try, I cannot get those to move back with the rest of the chopper. I assume that's why JJXB left them there. I wasn't about to re-do the entire chopper to make them work though when he did such a great job on it otherwise.