I have a lot of trouble picturing them pulling an elegant sequel based on what happened to the trilogy.
Like many have mentionned, if BioWare made a sequel, the two most likely options are :
1) Pick a canon, which would upset many fans of the series. Ultimately BW owns the IP and could do whatever they wanted with it, but the ME3 debacle really showed us that they must work with the fans to not tarnish the company's image.
2) Make all the major decisions in ME3 irrelevant by putting in their own continuity. This could work but hard to swallow for many fans, because they went from choice based gameplay in ME and ME2 to consequence based gameplay in 3. Whether you chose to kill or free the Rachni Queen in 1 was irrelevant for the story's main continuity : you'd still face and beat saren and sovereign. The enjoyment was in making the choice, feeling powerful in doing that, and giving your own flavour to your own playthrough.
That worked because most of the choices you made were a side gig compared to what was really going on and what you were about to do. I can see them having trouble side gigging the Reaper War, or Control, or Destroy or synthesis.
They could still do that, but the amount of contrieved sauce would be off the charts !
Which is why I think the most simple approach to making NME appealing to everybody is to keep the core elements we came to know and love (FTL, council species, alien sideboobs and lots of feels... and a spaceship with a French Beach name obviously) because that is what defines the Mass Effect experience and make it a complete standalone to the storyline of ME3.
And by standalone I mean having utterly NOTHING to do with the continuity of the trilogy's main arc. Like someone said, the scope of the main arc was so epic and so encompassing that it makes every other story of any other protagonist irrelevant in the grand scheme of the ME trilogy timeline. Which is not what you want when you want to have player agenda.
How can NME have nothing to do ?
- Alternate Timeline : not before or after on the same timeline, but a completely different universe. That means writing out the Reapers and the characters and finding a while new story arc to explore.
- Alternate Reality : as evidenced by the stargazer scene post credit, the shepard story could very well be a fairy tale within another ME universe (but why a grampa would talk about human intercourse to a young minor makes no sense and is quite creepy
). Or the infamous suggestion that ME2+3 were an unconscious dream by shepard when he got rubbled in ME1 (to illustrate)
- Alternate location : in order to make any event in the Milky Way have virtually no consequence, or be of anecdotical impact to the plot of NME.
These 3 ways are the only ones I can see working elegantly.