Return to the Milky Way!
Andromeda's explorers are shocked to hear that the Shepard 2.0 AI went rogue, and the Reapers are starting a new harvest in the Milky Way.
Revisit all the beloved scenery from the first three installments of the series, now rendered with higher resolution, weather and day-night-cycle provided by the fantastic frost bytes 3.5 engine!
Import your ME:3 savefile to have a rendezvous with your Shepard in the new climactic catalyst scene!
In a perverse way, and despite hating control almost as much as synthesis, I rather like this. I seems a very fitting consequence for the hubris of those Shepards that ignored every previous, disastrous attempt to control another species that occurred in the history of the ME universe. It's a rather good way to treat the endings actually - they "happened" but the consequences were not quite what everyone thought they would be.
You could even do something similar for synthesis. 6 months later everyone, organic and synthetic alike, is coming down with some hideous techno-organic virus that threatens to turn everyone into crazed zombies. Cue our trip to Andromeda in a desperate attempt to find a cure and change everyone back to normal. And everyone agrees never to speak again of that brief, but embarrassing, period when everyone was plastered in terrible, green, circuit-board tattoos.