In order to keep things simple, I thought a simple hex grid would serve as a decent rear cover for BX3e (like the one on the rear cover of the Gamma World rules from the same time period):
Maybe a map of the Grand Duchy of Karameikos would be a better option. Or…
Maybe the Expert rules cover on the rear cover. (?)
I think, looking at the three of them, that I’m partial to the map of Karameikos. Hmmm… What are your thoughts?
From the Sorcerer's Scroll 
1 vote for the Dutchy map.
The map would be cool, but I’m a basic kind of guy so simply black would also work for me.
Now there’s a third option… the Expert rules cover as the rear cover.
The Karameikos map as the back cover is really nice.
Expert rules cover on the rear cover is excellent!!
No big question: definitely the Expert Rules cover!
This is so very awesome!