After some thinking, I decided to share what has been on my mind lately. I know it's very speculative to the point of being nearly wild, but I've seen and heard of a lot of Science fiction stuff doing crazy stuff before.
The game starts concurrently with the first attack on Eden Prime. While the Normandy heads to fight the geth, an Alliance dreadnought carrying a lot of Alliance Marines, captured criminals of diverse races, like a krogan (from the commentary), and at least one former Alliance Marine turned aligned mercenary (the playable character) to either the Citadel or Arcturus Station for a trial when some unexpected wormhole appears sucking their ship in.
Emerging from the other side, the dreadnought and it's passengers find themselves in unrecognized space where they make contact with previously unknown alien races who either confront them in conflict or help them in exchange for help. After a while, it's revealed the wormhole sent the Milky Way people forward in time to the Andromeda Galaxy which is home to a powerful, normally benevolent government of species that fits The Federation trope more than the Citadel Council does with it's own Alien Non-Interference Clause to a degree while showing a monarchial image. Having evolved greatly with no Reapers, the balance between organic and synthetic in Andromeda is drawn similar to the original relationship between Humans and Robots/Reploids in the Mega Man franchise.
Overwhelmed by the extraordinary implications for themselves, their crew, and prisoners, the dreadnought's commanding officer, who acts like the Anderson to the mercenary's Shepard, puts the highly talented player in charge of their actions and dealings to find a way back home or establish a permanent colony in Andromeda. Giving them Spectre-like authority to get the job done. Doing some missions for "The Federation" along the way.
Along the way, it's discovered that the wormhole was technologically created by the main antagonist in their attempt to launch a large scale, genocidal attack on the Milky Way so the Andromedan synthetics won't follow their counterparts examples and rebel against their creators if and when any information of the Morning War and/or Reaper Harvests is shared. Having learned of Milky Way life from observation technology similar to the wormhole generators.
As the fight gets more intense, the "Anderson" character gets killed irrespective of the choices and leaving the player in charge of the Milky Way refugees, who becomes an official Spectre-like figure in Andromeda as a few synthetic uprisings begin. And thus, the story goes on.
I think it sort of fits with the spin off idea and if they want to look at some old trilogy characters, I was thinking they would use the observers to look at significant events during Shepard's time with their Crucible decision coming back to influence the uprisings for better or worse. Now that I think about it, perhaps such technology is responsible for the Dark energy build up and, on a temporal level, instigates the Rachni Wars.
Well, what do any of you think?