TL;DR: Art boards with transparent fills should continue to overlay the previous art board's underlaying board.

I can think of many use cases that have required a significantly higher number of art boards, assets and hours to accomplish than should be required. The feature I am requesting would address all of these issues in a simple and intuitive way.

Let's say I have art boards A, B, and C.

'A' displays my application, with 100s of assets, text, components, etc.
'B' displays a modal over the application in state 1, the default state.
'C' displays state 2 of the modal, which has been dynamically expanded to show a number of additional fields as the user fills out the modal.

Behind the modal (out of focus), there are 100s of elements that are on screen but remain exactly the same for all three boards since the modal is the only interactive element.

I am able to use an overlay transition between A and B no problem. So I design 'A' using a transparent art board to avoid having to duplicate the hundreds of items that remain constant between A and B.

Now here is the issue. I want to use an auto-animation between B and C to show additional fields in the modal. But I can't! Because doing so would make the underlying art board (A) that B is overlaying disappear. After the animation C will be displayed over a black background.

The workaround I use is to avoid the overlay transition entirely, and include a carbon copy of the contents of art board A in both B and C. This not only significantly increases the number of elements in the file but also makes any changes to art board A significantly more time consuming.

But wait! It gets worse. Let's say the modal isn't only accessible via screen A. Let's say there are screens A, D and E where the modal can be opened. Well now in order to have two different states of the modal, you need to recreate both B and C over top of A D and E. And just that quickly we are up to 6 art boards just for a modal that transitions between two states.

Here is what I am proposing:
Option 1 (Fewer art boards, fewer assets): Once an overlay transition is used, the underlying art board should persist as the 'background' for subsequent transitions until a transition is made to a non-transparent art board.

Option 2 (Same number of art boards but at least fewer assets): Allow an art board to be re-used as a 'background' element, so that the contents don't need to be repeated in other art boards.