User blog comment:James12708/Possibility of a Mass Effect 4/@comment-69.62.165.196-20121028063016/@comment-24174486-20121028154335

Three problems:

1. Just about everyone could be dead, especially if you got the minimal-EMS Destroy ending.

2. There's little potential for actual conflict, especially when Synthesis brings an Age-of-Enlightenment-of-sorts and Control has the Reapers serving as galatic peacekeepers.

3. The only "closure" such a game would provide would amount to "everyone does exactly what they said that they would do during the events of ME3".