I'd rather have more of what is, in my opinion, a fantastic game, than start off from scratch again. There's so much more that could potentially be done and they have a huge fan base, who I'm certain would buy more. Some ideas I've had include:
01. Resource-specific factories - replace the factory in their city (therefore each city can only have one of these); require the resource to be imported from elsewhere; only one of each type in the world (e.g. one rum factory, reliant on sugar being imported); in addition to the factory's production output, has a gold output, which is halted if the trade route bringing in the resource is compromised.
02. Just an idea to throw out there, but how about getting an advance that comes from an event that happens in the game? Something like "Military Tradition" could be researched in the normal way, but also could automatically be given to a civ based on a certain number of military victories, an earlier tech being researched and a bit of randomness thrown in for good measure. Guilds, similarly might be discovered after using a certain number of Great Merchants.
03. A mechanism (for modders) that does the following: upon creation of a mod for BNW, works out what content the individual user has, allowing them to play it within the confines of what they have. What I mean here is that if I create an Ancient world mod, and choose to have Babylon and Carthage in it, then players without the Babylon DLC and G&K will still be able to play it - but those two civs won't be in the game for them.
04. A "deluxe" version of the Wonders of the Ancient World scenario, with Assyria included and full civs made of Sumer and the Hittites.
05. A "Colonization" based scenario, or Deluxe version of the New World scenario in the Spain/Inca DLC, with an extra civ that takes advantage of the trait they were developing for the Pueblo.
06. One civ I'd love them to do (but don't expect them to do it!!) is Illyria with Queen Teuta as the leader. The Illyrians were the pirates of the ancient world and I'd love them to use their Lembi (ships) to interrupt and plunder those trade routes.