Another fun game mode would be a True Start Year Earth map, a la Crusader Kings III
Every piece of land would be occupied, either by one of the civs, with the approximate territory that they held in that year, or by Independent Powers, renamed for this mode if need be.
Instead of exploring, the whole map would be revealed from the start.
Moreover, starting in certain years would mean that you start already at war.
E.g., if you play as America in a 1942 start, you'd begin already at War with "Prussia" and "Meiji Japan"
We'd almost certainly have to wait until more civs are added to each age before this would be viable, but Firaxis should make it happen as soon as possible