A while back, I wrote about how, originally the game had an idea of "trade routes" as adjacencies between players' cities, but my good friend John Velonis (designer of, among other things, a really fun beer and pretzels game, Venus Needs Men) observed that these were a pain to visualize, and additionally weren't entirely historical -- trade routes were about access to resources that were far flung over a vast geography. This prompted an addition to the board of "trade good" spaces (the star spaces that show up in the image in this post) and of "caravans" (think Settlers road pieces) that players use to connect their cities to those trade goods. In the present ruleset, connections to trade goods are a scoring category, but connecting caravans to other players' cities or to trade goods let you increase your heritage and other players' caravans that touch your cities authorize you to hit them up for tribute.
Now the nice thing about this is that, somewhat like Knizia's Lord of the Rings, different player counts give a different feel. The more players in the game, the more potential cities you can connect to (more access to heritage) but the more exposed you are to tribute demands from other players (and of course the more players you can potentially collect from). With fewer players, you can still get heritage from your caravans, but you'll have to build longer routes to reach all the stars (at the same time incurring less tribute risk), or you can rely more on the other means of acquiring heritage.
But what took me a while to realize was that, for the first time, none of the core scoring mechanisms depend on the presence of other players, and in fact, it's possible to play the game solo. I've tested a solo variant a number of times now, and it seems to play pretty well. I'm hoping it will end up included in the ruleset inside the game's box. It's a variant because it does include two ghost civs, which exist merely to give you someone to attack (so military development is still a plausible path) and to give you some risk that you have to factor into your plans. The AI for the ghost civs is trivially simple -- when population grows, they add a warrior in each of their territories, and each time you roll a 6 when testing for the end of a generation, one of them attacks one of your territories. (Since the game lasts 21-24 turns, statistically you'll face about 3-4 attacks per game, and this feels just about right). They don't build or advance or anything like that, so you have to do the legwork of caravaning mentioned above by yourself, and there's no one to discover new techs on the tech tree for you, so scores are probably a bit lower on average, but it's still a fun way to get to play the game and a good way either to learn the game or to play around in the sandbox and try some new strategies.
I don't know if players who only plan to play solo would seek to acquire the game just for solo play, but I do think it provides a pretty similar experience with only a very few simple additional rules, so it has some value. And it would be neat to be able to say that the game seats 1-6 players; I don't think too many civ games can say that. Because the lion's share of the game time happens in the simultaneous action planning, the game length shouldn't really change much with player count in principle, but of course in practice 5 or 6 players will generally take longer to play than 1-2 players.