That's what I've been saying they should have done. As a jaw-dropping DLC would probably have been best. That way ME4 could have been a great game about the galaxy in ruins, the aftermath of the reaper war. I would have LOVED to see that!
To see the races struggle for power in a time of great chaos. How would everything shift? What races would come out on top? What happens to those cut off from supply lines after the relays stopped working?
I want to shape the world that Shepard saved. Or at least be part of it. Not necessarily as Shepard, though s/he makes the most sense for a leader people look to for advice.
Defeating the reapers was a huge deal, but the destruction they left in their wake meant millions of people starving.
They could have made so much out of this if they either canonized Destroy or the IT, which would have led to Destroy too.
Btw, if anybody is interested in reading the best Mass Effect aftermath story ever written, check out Reaper Dreams by a very good friend of mine. You will not be disappointed. It combines all three endings in a sense and shows how it could be done if Bioware had better writers. For all of you who want closure. 
Problem is that there are too many variables to take into account for this setting to even work, unless events of the Reaper war were canonized.
If someone made peace between the Quarians and Geth but they canonized the extinction of the Quarians because data suggests more gamers did that and the Quarians never made an appearance, that would ****** off a lot of people, especially Tali-mancers.
If the genophage is cured is made canon, and no imports can change that, but with Wreav as the head of the Krogan, that would likely get a lot of people angry.
And that's not even if we get into if everyone is now synthetic/organic combo, the Reapers are controlled by Shepard or not or if Destroy was made the canon and the Geth and EDI are all killed.
There is literally no way I can think of for the developers to take into account everything while also creating a cohesive story. And if they canonized the events of ME3, that would open yet another can of worms, and Bioware had a huge mess to deal with as a result of the endings.
Realistically, the only way the Mass Effect universe can continue after the events of ME3 is in another galaxy.