That is an interesting theory.
At the same time, with only two seasons left, things have to move pretty quickly. Cersei seems to be set up as Big Bad #1 for Jon and Dany to take down next season, then the final season will have to be all about defeating the Night King.
I think she only stands a chance with Euron and the Iron Islands by her side. For one, her title as the Queen seems shaky at best, and it's not clear how popular she really is in King's Landing. Further, their Navy pales in comparison to Dany's now. I also think it was foreshadowed a bit with Euron ordering the Iron Islands to build him 1,000 ships.
But I agree for the most part. It's going to have to come fast with only 13 episodes left, along with the Walker and Army of the Dead story line.
Of course, this could very well just be a story line in the book rather than the show, too.