Foreground Slide Master
So I though I would share something worthwhile here and see if it of use!
A possible Foreground Slide Master (with limits)
Sadly Storyline does not have a foreground master feature! If you want to have images or buttons overlayed on all slides you cant do it. Anything you place on the slide covers the slide master. So if you want the same object or shape in the foreground on multiple slides you must have many copies of it.
But there might be away I believe you can force shapes on master to the "foreground" in front of the contents of your slide.
I have tired the following method.
On your Slide Master you will need to add a layer called Foreground. Place what ever you want on the foreground here. Now on the Slide Master base layer add a trigger to show this layer. The triggers should be "On timeline start show layer: Foreground."
So now on your slides the slide master will show on the background and slide master layer foreground will show in-front of whats on the slide.
Now for the short comings. (you knew there were coming) You can't really uses layers on your slide once you do this. The slide layers will cover or hide the Foreground layer of the slide master.
Order of layers
I could untick the box to "Hide other slide layer" so the foreground layer is not hidden but this wont always do what I want. As this means none of layers can be hidden (unless I start using more and more triggers). And still the slide layers will appear in front of the slide master foreground layer.
Well it is part of an approach that I still found useful in a limited way.