I think a civ's vs. the world mode could be added pretty easily that might make things more interesting, even for good players. Basically, throw the kitchen sink at the players, AI as well, and have multiple different master quests that are the only way to win (so each game is not the same master quest). Monsters can stream out of the wildlands, events can pop up and terrorize civilization; basically just eliminate the checks that stop monsters from being a real PITA (such as no deliberate city-razing attacks). Difficulty levels can differentiate different levels of havoc and monster spawning, so fun can be had by all.
The fact that some simple tweaking would essentially create the game mode (no war declarations, or at least no empire empire wars or kingdom kingdom wars, tweak monster behavior) doesn't mean it couldn't become an all-out awesome feature in and of itself with some loving care. Some dialogue that goes it with (I imagine the AI civs will drop like flys at aggressive levels), some special negotiations that take account civilization itself could be lost (trades and "surrendering sovereigns" to the player), some events and quests especially for this mode, the right monster aggression levels, having wildland clearing being a necessity (stop the monsters from rolling out), and other neat ideas I am sure others can think of would make it a mode some people may play exclusively. This game is versatile enough to accomodate it, and it is another thing this game could do that others can't. I would buy it as a further expansion, though I would probably buy any expansion ideas for this game.
Quests should be easy to make. If they don't have a quest maker that can churn these out as fast as you can write them, then they need to invest in that tool. The art assets are a factor, but really reuse here is completely understandable. This way you can get several (or more) master quests which make this player vs world games more unique and fun. You only need one to win, but which one will be required in the current game will be a mystery until you build up to solve it.
The civ's AI is not really going to satisfy hardcore players here anytime soon, so this mode would be an easy to implement way to appease players who are good and player's who are not very competitive(since they will essentially be cooperative). So casual players may be tempted in to the game because they will be working with the AI to bring civilization to the world (maybe less stress). I for one would enjoy it because that means you never really have to hold back at trying to win once you really start to get your civ going to enjoy the fruits of what you have built up. The late game tech and weaponry will come in handy as the world doesn't pull any punches at you (and this AI can't cheat because it's not supposed to be playing the same game!).