BioWare has announced more details surrounding their ME3: Extended Cut and ME3: Resurgence DLC slated to come out later this summer.
At the PAX East panel today, the development team went into a bit of detail regarding the Extended Cut DLC. They mentioned that it will not be a re-imagining of the endings, or a new ending, but a number of things to wrap up the ending in a prettier package. According to the team, the DLC will contain more than just a few cinematic scenes - it will be a considerable amount to provide closure. The development team also mentioned working on a lot over the next year in addition to the content announced this week.
They also showed a new trailer during the panel that details some juicy bits of the Resurgence pack, including the new Batarian and Geth characters, fancy weapons, and a cool new map that looks wet, dark, and badass (wait, what am I describing here?).