We all knew this would come, they even pretty much said so during Mass Effect 3 interviews when they said things along the lines of:

Whilst Commander Shepherds story is now complete, I don't believe we're done with the Mass Effect universe there will always be more stories and options to explore within it just without Commander Shepherd as a character.

What probably will happen is you'll get an interluding story cutscene based on your choices in the original trilogy before having a new story that will fail to deliver any real difference in the ending. Considering the various elements of the galaxy united behind Shepherd it would not be hard to believe that a Salarian - Krogan war soon came to be within the Galaxy (especially if you were renegade) With the obvious collapse of the council and the citadel's destruction we could also expect to see many more issues. Or perhaps the citadel would be rebuilt. Either way, the change or control endings really provide the worst possibilities for a new story set after the original trilogy. Combine because it has everyone basically the same and Control because Shepherd becomes a peace keeping defensive force when he takes over the Reaper forces.