ME3 ending messed up the Milky Way for any future ME game......well other than a pre game or something going on during the later parts of ME-early ME3. But anything after ME3 would require a cannon ending thereby ticking a whole bunch of people off.
Synthesis(aka StarChild magic ending)...every living things was basically networked together according to what I remember, PLUS if Andromeda team came back, then they would be the ONLY pure biological group in the MK galaxy...thus almost a different race even among their original racial groups. Since all plants, animals, and races were combined with tech in Syn ending, there is even doubt that non-combined people could survive due to a lack of food, and possibly hostile reactions to micro-organisims who should also be combined. Since the Reapers were also involved in this, then you would have the combined knowledge of all the Reapers races back millions of years to sort through and integrate into modern technology, thus radically changing everything especially in 600 years time. Since eveyone is linked and the same, all conflict should be nil and a really boring place for a game.
Destroy...my cannon....ok, is it low or high destroy?? Low would possibly take thousands of years to fully recover with much of human and other races being destroyed, thus not much left to come back too. High destroy, would be best, allowing full recovery, lack of AI, Geth or Reaper based.... Which the Geth angle still makes absolutely no sense, due to GETH BEING SOFTWARE!!!! Geez, Bioware forgot their own story, since the red god wave shouldn't be able to distinguish between one 1010110 and 0101011001 which is all the difference between a Geth and a Suit repair program....Geth were living in and helping out Quarian bio-suits, for goodness sakes... But then you have possible wars, between Krogan, random new race, maybe a newer AI going wild.....
Control, there should be enforced peace by Shepchild and possibly strafe and resentment....or he is Emperor of the Galaxy by this time, no telling, but by no means should they be gone....
Oh, and don't forget Walk away...reapers win and the cycle starts again....
There isn't a good way to handle all these options with just a handwave of oh, it has been 600 years.....which both the Asari and Krogan live longer...maybe everyone in StarChild magic ending....because in two of the endings, you have peace, and Reapers, one you can't probably live off of the supplies it would give, one possible to still be in ruins, one possible in war or wiped out by AI, one ending you would been seen as a possible threat to his control, one ending you are a threat or lesser race due to being pure Biological, and one ending where nothing of the old worlds exists and it is a new cycle...
Yeah, leave the MK galaxy as the burning trash heap that Bioware created and walk away to the new better and still open to new ideas Andromeda Galaxy. I wish people would stop pining for the old ME galaxy, cause Bioware put paid to it, and that Trilogy is done and they burned the bridge behind them. Do I wish it wasn't this way....yeah, they could have done a much better ending, much more lore friendly and made a future possible, but they didn't and thus without stepping on many feet and burning many choices they are stuck.
Now will they burn those choices...maybe, they are the ones who came up with the stupid idea anyway....I still think if you want a future in the MK galaxy, then they needed to make the indoctrination theory cannon and make different endings....Shepard wakes up on a lifepod after the destruction of the Normandy, and all of ME2 and ME3 was a dream....
LOL, that would chap some people, but to me it would still be better than the ME3 endings.