BioWare already made some fairly large changes to the ending of Mass Effect 3 in response to fan complaints, but more might be on the way. Some script information buried in the Extended Cut downloadable content seems to imply that playing the upcoming Leviathan DLC will make even further changes. Spoilers follow.
Eurogamer reports that the Leviathan race had a hand (or tentacle) in making the Reapers, echoing the theme of organics versus synthetics present at the end of the game. If you play through the ending after playing Leviathan, you'll be presented with additional dialogue options at the end, to learn more about the Leviathans' untimely demise.
The full ending dialogue options were extracted from the Extended Cut and posted on the BioWare forums. Click through if you're curious about the additional dialogue, but not curious enough to buy a DLC pack and then play through the ending again.
Keep in mind that this is second-hand and through file extraction, so it's impossible to tell what will make the final cut. Shacknews has contacted BioWare regarding the changes and will update as more information becomes available.