Canonizing an ending has always been an option, but everybody would rather whine about the lack of meaningful choice. Also if TW3 is any indication, choices carrying over to the sequel being a selling point is dead and buried.
It has always been an option and has been suggested by fans occasionally at least since the release of the Extended Cut, but The Witcher 3 really demonstrates that it is viable way to go while still crafting a critically and commercially successful game. TW3 largely did what some fans have been suggesting the ME series do regarding the vastly diverging endings, and just role with a sole default import, and they've made it work. Not many people cared because it was a great game.
GalacticWolf5, on 09 Jun 2015 - 1:25 PM, said:
They've already confirmed that there will be no canon ending to ME3.
Sole canon and sole import need not necessarily be the same thing. If an ending is appointed sole canon, it means that the other ending choices don't happen officially, sort of like the ending where Shepard dies in Mass Effect 2. That is quite different from a sequel that explores the results of only one ending choice, but isn't accompanied by dev statements saying that chosen ending is the sole canon. Basically if you chose any of the other two endings they are still canon for your version of Shepard, you just don't get a sequel that explores the results of that choice.
If Bioware goes that route, and realistically I think it is their only viable option for a sequel set in the Milky Way, it might also be a good idea to have the fans vote on it like they did with Fem Shep's appearance. Set up a poll before hand where fans are asked if they'd rather see a Destroy, Control, or Synthesis sequel. It gets fans involved in whatever path the sequel follows, would probably damper some of the pre-release complaining, and would provide some hype and excitement for that game before the basic plot is even announced.