BioWare has just announced Mass Effect 3: Extended Cut, and confirmed that there will be no new endings added to the game. The original news from a few hours ago put it as though we should expect some new endings, as people weren't happy with what was on offer. The studio has said:

BioWare strongly believes in the team's artistic vision for the end of this arc of the Mass Effect franchise. The extended cut DLC will expand on the existing endings, but no further ending DLC is planned.

What BioWare will be doing, is expanding on the ending, they explain:

By creating additional cinematics and epilogue scenes to the existing ending sequences. The goal of these new scenes is to provide additional clarity and closure to Mass Effect 3.

Mass Effect 3: Extended Cut DLC will be made available sometime "this summer".