For the last few weeks Civilisation V always generates the same kind of boring map: flat land, very few mountains, most of the land desert or tundra. Is there anything that could have caused this? I have not installed any mods or modified the game in any way.
Did you configure it to be arid and old; or random for everything and just have a run of similar worlds?
You could explicitly set the World Age and Rainfall (from your description, the temperature could have varied a bit) to mix it up.
Configure the settings to a younger earth (3 or 4 Gyr) and temperate or wet rainfall if you want to get away from what you're describing. You can do so on the "Advanced Setup" panel when you configure a game.
If you are into mods, use the Communicatas map. It will divide the world into two large continents with several small islands.
You can also save it and add it to your game's folder so it is available fo rnon-mod play.