I would expect:
-One "game-changing" mechanism: either something we've seen before like religion, espionage, corporations, or maybe something completely new (completely revamped diplomacy model? Evolving city states?)
-A few new civs, or possibly new leaders for existing civs
-More balance tweaks, to go along with more wonders/buildings/units/techs
-Global quests/events, like BtS
I think an expanded city-state model would be a great thing for an expansion pack. There's been tons of ideas for elaborating on the city-state diplomacy, so it wouldn't be too challenging to bring a few new types, a few new methods of gaining and losing influence, and a few more quests with them.
Throw in some new balancing, maybe a bit of re-jigging the air and naval games, and overall I think it would come out well.
-One "game-changing" mechanism: either something we've seen before like religion, espionage, corporations, or maybe something completely new (completely revamped diplomacy model? Evolving city states?)
-A few new civs, or possibly new leaders for existing civs
-More balance tweaks, to go along with more wonders/buildings/units/techs
-Global quests/events, like BtS
I think an expanded city-state model would be a great thing for an expansion pack. There's been tons of ideas for elaborating on the city-state diplomacy, so it wouldn't be too challenging to bring a few new types, a few new methods of gaining and losing influence, and a few more quests with them.
Throw in some new balancing, maybe a bit of re-jigging the air and naval games, and overall I think it would come out well.