Due to the overwhelming number of fans who were disappointed with the ending to Mass Effect 3, BioWare decided to create an Extended Cut Ending. This ending would flesh out some of the holes within the current ending that confused gamers. No one has any idea how this new ending will play out, except BioWare, but now we know who will be part of said ending. Admiral Hackett, EDI and Kaiden will all be included in the DLC. Confirmation comes from their voice actors Lance Henriksen, Tricia Helfer and Raphael Sbrage respectively. You can expect to get your hands on this DLC this summer.
On a side note I wanted give my impression of the overall game but I've decided to wait until the Extended Ending before I publish those impression. Although I don't think the extended ending will change my opinion too much.