I think it all depends on what the storyline is for the game post Rohan. I suspect is isn't going to be as simple as "take part in the Battles of the Pellenor Fields and Black Gate." If anything, I think would be fairly logical for Gandalf to task us off with warning Erebor of the coming assault and assist in its defense. Essentially protecting the Freeps backside while Gandalf & Co. handle the Ring business, as we have been doing ever since the beginning of SoA.
But I do agree that the overall momentum of the game has been, and should be, heading towards Gondor and then Mordor. That's what the majority of the player base presumably wants and expects,and Turbine obviously will need to be careful with how it handles the flow and pace of the game toward that end.