There are two scenarios I would like to see. One of them, if you notice, sort of already exists, just needs tweaking.
-The First is 1984. Three playable super civs, Oceania, Eurasia and Eastasia. I imagine this would mean an overhaul of the civic functions to reflect Ingsoc, Neo-Bolshevism and "Death Worship." As for the unplayable civ, make the disputed territories these superstates fight over be modelled after the indy-cities of RFC. Obviously tech would need altered as there cannot be certain technologies like computers and such in the 1984 world. Rocket bombs, floating fortresses, come on people! In a way this sort of exists as Next War, but still...
-The Second is something that hit me the other day, maybe not so much a scenario as a map, or maybe it is a scenario. How about a post-nuclear war world? A large Earth map, post-nuclear civs (for example, fractured U.S., fractured Europe, etc), arid tiles covered with city ruins and nuclear fall out. Just something to think about.
All scenarios to be played within BtS, most current version, well last version than in this case I guess.