Bioware's gone on record saying that any DLC will be set "during or before" the events of ME3, so that means no alternate ending. Then again whatever this DLC is, they're being really tight-lipped about it, so who knows, they might be full of shit.
I mean, I enjoyed the game, and I love the multiplayer (maybe even more than the singleplayer) but there were SO many things wrong with that ending. I mean, just logically. though I did read the infamous "Indoctrination Theory" on the ending and it does make a lot of sense, though I don't really think that Bioware's smart enough to have actually done that.