You don't think there's a lot of that universe that was still unexplored before they razed it all to the ground?
The main reason I'm good with them doing a parallel story is because they
did build up so much interesting lore, then turned around and leveled everything. Now we're potentially left with yet
another post-apocalyptic science fiction game where humanity's sifting through the ruins and rebuilding itself, except this time on an intergalactic scale. Maybe they can bring back the husks in a post-ME3 game too, so we can get the
full "space zombie apocalypse" effect.
That's how I felt 2 seconds after I finished ME3 and continued to for several weeks after. But really everything is still there. They blew it all up, but they didn't really blow it all up. As terrible the ending was and the whole 3x space magic I actually prefer the original ending to the EC because the destruction of the Mass Relay System opens up the entire galaxy and sets up the world to be a very interesting place post Reapers.
The entire MRS was made by the Reapers to put advance life down the same path and steer them in predictable ways. They became dependent on the Mass Relays for practically everything and their entire foundation of technology was based around Mass Effect fields. When you take all that away it forces these species to think outside the box and come up with new technologies and forms of FTL travel. Most especially if the majority of those species are stranded in a foreign solar system tens if not hundred and thousands of light years from home.
Even with the EC they could fudge it so that many areas in the galaxy were stranded for decades after the end of the ME3, some areas were able to repair their Mass Relays within a few years or so and a core network of systems reconnected around the Earth and Galactic armada, but many others only reconnected with themselves or were just completely cut off.
If you set ME4 say 50-100 years after ME3 ending that leaves a lot of time for many things to change in the Galaxy and for some rebuilding, but it also allows for the possibility of a lot of interesting developments as well. Cut of Asari or especially Turian, Salarian, Krogan and Quarians/Geth settlements and planets could turn into very different and distinct places. Cut off from the rest of the Galaxy they wouldn't know what if anything had happened. And depending on their resources, survivors and such could develop into very unique and interesting societies in the face of total isolation
ME4 could then be about using new FTL technology to reconnect with these lost sectors and systems, bring them back into the Citadel fold, though some might not care to, and to explore new areas of the Galaxy that were previously not easily accessible due to the fact that the Mass Relays only gave quick access to a small portion of the Milky Way, leaving hundreds upon thousands of totally unexplored star systems.
There is just so much potential for a post ME3 galaxy to go through. The focus could be on putting a broken galaxy back together and the fallout of a near galactic annihilation and how certain areas of the Galaxy could have rebounded better than others, shifting attitudes and philosophies based off the fear of another Reaper type threat that could be out there. Or it could be about breaking away from the Reaper's mold of how galactic civilization should work and exploring new areas of the Milky Way and discovering intelligent life that developed totally outside the Reaper influence.
Jumping to a new Galaxy is just cheap because it allows them to just leave all these questions of how the Milky Way transitions from a post Reaper world and the realities of how different cultures and species would evolve after such a traumatic and devastating war.
