Have the game take place in a part of the galaxy unaffected by the Crucible wave. Connect that part to the places of the trilogy via one primary relay and have that relay conveniently destroyed prior to using the Crucible. The wave doesn't reach there, the races are already there (since the region is already settled), there is room to have quarians (Idenna, Deception was confirmed faulty by the developers) and the geth - some isolated pocket of them, stuck in that part when the relay was destroyed.
Forced to find new trade partners and resources, the races start to explore previously uncharted relays.
- I'm assuming this previously settled region was on our galaxy map during the trilogy - otherwise, it's addition is sort of a retcon.
- The reapers have apparently not yet been to this region, so it should show up as reaper-free on the galactic map through ME3.
- Destroying a relay does a lot of damage. It is also the sort of event you would expect to be reported / known - though I suppose reaper destruction of comm buoys might keep that event unknown to the rest of the galaxy.
- For this to work under a Control or Synthesis ending, the 'friendly' reapers would never visit this region. I think a lot of players who chose Control or Synthesis would expect the reapers to repair / replace / maybe build more mass relays.
- It canonizes the idea that the Crucible wave has an AOE around mass relays, and would not impact the entire galaxy.
- It would have to take too long for them to get to another mass relay - or for anyone else in the galaxy to get to this region to be feasible - so they would be completely isolated for the duration. Also, no comm buoy restoration.
- So they develop the technology to travel to Andromeda and set forth - for what reason? Wouldn't they try to get in touch with the rest of TMW galactic community first?
I don't know that setting it in a previously settled but somehow completely isolated part of TMW would be any less contrived than traveling to Andromeda.
And I haven't heard a compelling argument about why it should be in Andromeda. As for why it shouldn't be there, because getting to Andromeda will require either a heavy dose of space magic or creation of new lore (and retconning some old lore as well).
How about... because as long as we're in The Milky Way, people will be looking to reconnect to those old familiar people and places. The galaxy map showed us mass relays and settled (and unsettled) worlds in every part of the TMW. Couldn't the folks from the isolated region get to those areas a lot quicker and easier than going all the way to Andromeda?
As for the tech, well - writers have a tendency to create whatever tech (or space magic) they need to tell the story they want to tell. Throughout the trilogy, we've been stumbling on previously unknown tech, experiments, artifacts, etc., that were not part of lore until they were.
I can appreciate that some of the resistance is based on tech / lore issues. I also suspect that a lot of the real reason for the resistance is the desire to revisit familiar places and see the results of some of the choices made in ME3, and I think sometimes it's easier to just bite the bullet and gth outta dodge instead of continuing to allow people to feed their hope of ever returning to previously tread territory.
Also - it covers their (Bioware's) backsides. Announcing a new game in TMW would engender expectations of reconnecting with people and places from the trilogy.