The Ballad of Gay Tony
| This article concerns a future game or content that is unreleased.
Some details may be speculative, and may not turn out to be correct.
The protagonist is Luis Fernando Lopez, a part-time hoodlum and full-time assistant to legendary nightclub impresario Anthony "Gay Tony" Prince. The storyline will interweave with the original game and first episode, most notably the Museum Piece mission which includes all 3 protagonists and secures the "Impossible Trinity" achievement.
Rockstar says that "players will struggle with the competing loyalties of family and friends, and with the uncertainty about who is real and who is fake in a world in which everyone has a price." According to Rockstar's Official Website, "The Ballad of Gay Tony injects Liberty City with an overdose of guns, glitz, and grime."
The teaser trailer of the DLC, entitled "You'll Always Be The King Of This Town", shows Niko Bellic (with his off-the-boat clothes) and Gay Tony during the confrontation on Colony Island dealing with Gracie Ancelotti and the diamonds.
In addition to this Rockstar has also announced they will be releasing Grand Theft Auto: Episodes From Liberty City at the same time, which is a disc found in stores that will include both episodes and will not require an original copy of GTA IV.
Sam Houser, the founder of Rockstar Games, said before its release "Liberty City is the most vibrant game world we've created yet. The episodic structure has allowed us to interweave stories, gameplay and atmosphere in a whole new way. The team at Rockstar North have yet again surpassed themselves, and made something that is both epic and very innovative. This episode's focus on high-end night life contrasts with the biker gangs portrayed in The Lost and Damned, whilst giving us a lot of new gameplay possibilities." 
Note that the game logo, unlike that of The Lost and Damned, omits the "IV" from "Grand Theft Auto IV".
The Ballad of Gay Tony Official Website
The Ballad of Gay Tony Official Trailer #1
The Ballad of Gay Tony First Official Trailer on Youtube