The makers of the Grand Theft Auto modding tool OpenIV have announced an ambitious plan to bring Liberty City, the sprawling metropolis of Grand Theft Auto 4, to GTA 5. It won't replace Los Santos or Blaine County, however, but will actually exist alongside it, on the other side of the sea.
There aren't any proper trailers or screens just yet, nor is there any mention of a launch date, although the FAQ states that the developers are trying to get it out as soon as possible. Whenever it happens, because of both the sheer size of Liberty City and concerns about distribution rights (which they don't have), the mod will "convert" the city from Grand Theft Auto IV into GTA V, and thus require both of them to operate.
The mod will work with Episodes From Liberty City, but you won't be able to hit those streets in multiplayer: Rockstar, you'll recall, doesn't allow mods in GTA Online, and there's a "high probability" you'll be banned if you try. You'll need a lot of free hard drive space for the converter to do its thing, and possibly a good chunk of time too, although specifics on that will have to wait until the mod is released.