There has been nothing for level 50 players to do for a very long time, just accumulate and store resources which isn't challenging or fun. Earning rare buildings, generals, completing all the quests isn't enough. The game needs more content for a longer lifespan, currently its just 6 months for an active player.
How about monthly events similar to the crisis quests, not just the seasonal events? Expand the island to the ocean around it, perhaps an underground area could be used too?