I believe that, the overall concept of "Enemy of my enemy is my friend" will come into play.
If Walled Eldians would make any pact or alliance with another countries, then this could happen. Apparently nobody likes Marley Country, so that would indicate a good point of interest, the main problem is that it seems that everyone in the world thinks of the Eldians as demons, so nobody would never likely to become their ally, first they should surpass this situation.
If/If not, they can overcome this situation, there are thousands and thousands of Collossal Types trapped inside the walls.
That alone would mean the entire world would become nothing but ashes. As normal-dumb titans seems to never be out of energy, except during night time, the Collosaal Types inside the walls, would never run out of steam also. They alone would be enough to conquer the continent,
Eren needs to dominate the Coordinate first, and with na enough big dosis, apparently any Eldian could become colossal type.
If you dress up those colossal types with heavy armours and give them weapons(something like we saw on cp. 92), hell is unleashed and they will never stop.