I dont believe it will happen officially. As far as I understand it is balancing issue. The game contains some limits of object it can handle (If Im correct some of these are limits of the Unity engine itself - data types etc).
So for example you can have limited amount of the cars on the streets. Limited amount of the pedestrians on the streets. Once you reach this limit, the game stops spawning new "agents" until some other reach its destination. The whole game is balanced that if you play on 9 titles and dont try to "powerplay" (reach some records numbers of population) these limits are not a problem and wont break the game. But if you fill the 25 titles or all 81 titles, these limits will be gamebreaking. You will have empty roads (the same amount of cars will spread around the larger area). But the game will not be able to generate new ones. So people wont reach work. Business will not receive goods. Truck wont pick up trash, etc. Most of the city will collapse because 16 000 cars can cover 9 title city. But 16 000cars wont deliver services around 25 titles.
Because it is engine limitation it is not so easy to change. (new engine is needed)
There are mods for this on the PC that allows you to build on more titles. If you are willing to get your game broken (to have huge cities for screenshot) it is your "problem". But Im pretty sure, they wont introduce this officially.