Suggestion: auction off cities abandoned for two years or more, but keep a record in case the player comes back.
Why? New players cannot catch-up with the established players. Therefore there is little appeal to new players. A new player, once completing the general tutorial, could bid on a city up for auction which would then let them compete with established players. The winning bidder would get to take over the identity of the city with its bonuses and transfer in their existing troops.
Otherwise the game remains a slow attrition of existing players.