Modders are working on bringing Liberty City from GTA IV into GTA V.
Leave it to GTA 5 fans to make their own singleplayer DLC if Rockstar doesn't. A team of modders has decided that Los Santos and Blaine County just aren't big enough for them to mess around in as Trevor, Michael, and Franklin, so they decided to take Liberty City from IV and plop it right into the newest installment.
This isn't a mod where the authors manually port over the assets from the previous game and painstakingly place all the objects. This isn't just a graphical facelift, with the older city being loaded up in GTA V's engine. This is more of a technical experiment, really.
Conceptually, the mod is actually more than your typical mod, which would simply affect the game files of GTA 5. This project seeks to build a virtual bridge, if you will, using OpenIV between Grand Theft Autos 4 and 5. To function, it will require both to be installed on the system running it.
Basically, it will take the files for the city from IV then convert and install them in V as if they were parts of a singleplayer DLC. The entire map of GTA IV will inhabit the same physical space as V's island, dropped into the sea next to it.
Liberty City will not replace Los Santos or Blaine County, but appears across the sea. Using the power of OpenIV and openFormats, we were able to bring Liberty City right into GTA V's map.
The team behind this converter tool happens to be the same guys who came up with OpenIV several years ago, a modding tool used initially for GTA IV, and later for Max Payne 3 and GTA V. The team is dedicated and experienced, making it pretty much guaranteed that this project will impress when released.