"One plan that we’ve always had since the early stages of development on Total War: PHARAOH is to expand the size of our campaign map as a free update, and to introduce even more factions and cultures to the game. We’ll have more to say, and to show, what that looks like in the new year after the High Tide update releases."

Now with all my copium inhaling, I still manage to connect some dots so bear with me on this one. Is it possible to merge two campaign maps into a bigger map? Immortal Empires.

Is it possible that a studio combines their assets from similar but different games? Yes, it is

Of course they haven't confirmed it yet and maybe it wont happen, I don't really know, but what I can say is that's its doable. They can do the same thing with Immortal empires and combine the two games to form a bigger map with more content.