You can purchase a limited number of premium expansions (500 diamonds per expansion.)
You can complete a certain amount of provinces. (Once you complete a province, a message will pop up that tells you how many more provinces you need to complete in order to unlock the next expansion.)
You can unlock province expansions in the technology tree (this is where you find different things that are currently unavailable to you, but you can find out what is required to unlock them. Just look on the technology tree and you'll find city expansions and what you have to unlock first.)
That's it, I believe. Most of mine come from option 2.
Looking at the overall city expansion area I get the impression that the maximum area your city can cover is 64 x 5 x 5 squares. According to the wiki it is possible to get 64 province expansions. Adding the 20 possible city expansions and the 15 premium expansions makes 99. Where will they all fit?
There are only 64, and not 99, if u choice the premiums, u will only be able to get fewer "i other ways" , but maybe the map will get bigger in time, who knows...
yes it does seem a bit of an impasse once you reach 64 blocks available, as once all tech is unlocked and all expansions got, there is no need to produce anything more for yourself, except maybe helping out in your fellowship.
Maybe once you complete as above, you get the chance to start a new town, where you cant trade between yourself, or something like that.