In games like Pocket Legends and Arcane Legends, friends can work together to complete a mission. Suppose we had that in Battle Dragons.
We can have really hard PvE missions that one person cannot complete. But, two tribe members working together can clear the missions. And, reload the resources so that we get some loot for each run, at least to compensate for the cost of building the troops.
Also, if we have repeatable PvE missions, perhaps some smart programmer can figure out how to add some variation instead of using the same static maps. What if the game randomly chose from existing player maps to use in PvE? Some math could be done on player maps to determine level of difficulty, and appropriate level maps can be used in PvE.