I don't really want an AU, seems a waste of a good U! But at this point it seems as likely as cannon ending sequels successors or prequels.
The whole "It does not have to happen before, after or during the me trilogy" could just be a troll misdirection of course.
If there was a cannon successor game, they would not have to pick the "best" ending for it (which is up for debate oc).
Metro 2033 has 2 endings (spoilers)
Nuking the mutant Dark Ones is the ending that the next game bases its plot around, while the slighlty harder to get one (where you realise they are not a threat, and befriend them) is probably a lot better for the world the characters inhabit, the lack of conflict invovled in this scenario would not make for an interesting sequel. (note don't draw parralels to Me3 as the dark ones are NOT the reapers!).
I.e the universe needs to be a messed up place to make an interesting dramatic game. The alternate ending to breaking bad episode 1 where Walter White accepts his friends charity and gets no strings attached cancer treatment would not be an interesting 5 seasons.
I don't think Control would be that fun, as most stories would go: "we need a hero to stop this new threat! ***Reaper shep, I got this one you guys, assuming direct control*** Wow that was easy". Or Shepard going bad after hanging around for a bit too long and not running things for everyones best interests any more.
Or Synthesis, if it is as benevolent as some speculate, then there is no more conflict and everything is dandy... Again, perhaps a good idea, but not something worth playing.
Destroy. The Galaxy is in a right state, people are cut off from each other, some technology damaged, some written off. Who knows what the timescale for a complete relay network fix will be. Many directly invovled in the war effort will have found comradeship with their alien allies, but what about the colonists in the back end of knowhere, who survived the reapers by the skin of their teeth? They won't be seeing the victory flotilla for months or years, petty warlords and pocket empires will rise and fall, and they may feel abandoned by their barely functioning leaderships.
Will they rejoin the fold when the time comes or resist? Will the systems alliance and alien leaders allow this dissent or crack down?
Of course I could be talking a load of quads.