The rules for a coming up with a codename are the same as before: suggested names must somehow relate to the previous version's name. According to the guidelines:
Cambridge → Leonidas →
The link between Cambridge and Leonidas was "both are ships of the United States Navy." The link between Leonidas and the new name must be different than that link, and different from any other previous link.
There are already numerous suggestions being bandied about, including my favorite, a nod to the fantastic sci-fi movie, Firefly: Like Leonidas, "Malcom was also a badass leader who fought against a much larger army."
Now that you know what the Fedora community is looking for in a codename, be a leaf on the wind and come up with a suggestion or two of your own.