The reapers have returned from dark space. With Earth under attack and the mass relays in a state of chaos from refugees and the councils fleet engaging reaper forces the galaxy is a mess. The outcome is unknown, the warfare is devastating. The galaxy will never be the same.
This came almost exclusively from my imagination. One can only use reference material and concept art for so long before you run out. This is my spin on a frigate shipyard for Cerberus.