I want to build a slot machine project, and I need it to be dynamic.
I want to be able to control each time what three icons will be displayed when the wheels stop spinning.
I don't want to build it in the traditional way with three vertical strips of all the icons and then animation the Y position, I want it to behave like a particle system that generates instances of the icons on the fly, and knows which ones to pick according to the outcome I choose.
I don't see the point in the instance-generation approch.
You have to pre-build the icons in every case, so why not pre-building all icons and map them? If the desired icon is a circle, you can translate "circle" to any position, layer index number, array index, or however you build things up.
Keep in mind that an Expression for such a purpose will be fairly complex, might executing very slow due to the "hold value" problem. Evalutate if keyframing will be the better and faster solution.
There is also a cross-way where you would keyframe animate all posibillities and use expression to "choose" the corresponding animation.