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.
Before you guys go freaking out. Both the canonical and my versions will be included. The bulkier style ships will require certain tech research towards that end there are merits to both designs that should be used.