This one's a bit of a weird idea, but bear with me. Basically, I'd like to see a polygon fill layer effect that reverses the fill, globally covering the area around a polygon with a hole where the polygon is.
I came up with this thought while applying some of the design techniques in a couple (awesome) John Nelson posts here and here, where he created a visual mask for a map by creating a layer showing every state except Wyoming in a transparent white over a basemap.
In my case, the map I was trying to create didn't have any polygons except for the AOI I wanted the mask around - there were no other polygons to create the mask from. I got around this by taking a global polygon dataset in another Cartography blog post, copying it to my organization's SDE, and creating a Query Layer to generate a polygon covering the entire Earth minus that outline and drawing the mask. During this process I thought it would be so much easier if I could just do all that with a layer effect in Pro.