If there is a setting combination that would create this I'd like to know, but it doesn't seem there is one. Will it be possible to add a feature to force generated dimensions to be surrounded by void like The End? With a configurable range for size?

I'm thinking "pocket dimensions"

Of course I don't know what that will do when multiple portals are used. It might have to force one entrance in that case.

there are dimensions which naturally use the End's layout of "island in the middle, ring of void, islands all around". however, under no circumstances do these dimensions force player entry to the center island; all dimensions respect the scaled linear mapping of coordinates like between the Nether and the Overworld.
it would technically be possible to implement the thing you're asking about, but right now the situation is as described