This, I think, is an interesting idea. Throughout history, there were specialized forces, just and many players specialize their cites in Grepolis. By themselves, those specialized forces had weaknesses. However, they would meet up at rally points with general armies and/or navies or with other specialized forces to combine into a cohesive whole. I think it might be cool to have rally points in the game, where players who have specialized cities could combine the forces of their own specialized cities into a larger, more versatile force. It could be set up so that the points are one per farm island, for example, and only that player's alliance controls the entire island can that player use the rally point. They can send their forces from other islands to the rally point, as long as this island remained completely dominated by their alliance, and then send the combined force off to attack. The conjoined armies would separate back into their parts to return to their home cites once the attack lands. There could be a limit on the number of cities and/or a population limit on the composite army a player could use. I think this would be fun and add a little something extra to gameplay.