Fallout Miami is an upcoming Fallout 4 mod that creates an entirely new world set on the coast of post-apocalyptic Florida. When finished it's expected to be about twice the size of Fallout 4: Far Harbor and packed with new quests, locations, and factions. The mod isn't complete yet, but in the meantime you can take a look at an early version thanks to modder Mika999, who uploaded it to Nexus Mods yesterday.
Bear in mind this early version isn't reflective of the final version Fallout Miami.
"This early version of Fallout Miami is the first worldspace I made when I started this project," reads Mika999's post on the Nexus Mod page, "and that's why I must emphasize that it won't be present in the final version of Fallout Miami and is not associated with the current project."
Essentially, when the Fallout Miami modding team grew, it was collectively agreed to make a number of changes to the world in order to improve optimization and provide a higher level of detail. A new worldspace was created for the mod and not everything from this original version was carried over. But Mika999 didn't want to see the original version of Miami be lost forever, and decided to release it so players could take a look at it for themselves—sort of like releasing a first draft of a novel.