Yes, maybe some upgrades/disasters, or add a 'pollution' component to the buildings that builds up & needs to be dealt with in the latter stages? It's a tough one but I think maybe if you constrain the size of the map it'll become easier to implement some solutions to it? Like having older trees that count for more, but needing to build 'cleaning' stations next to buildings that might force you to clear away the trees at that spot, or something along those lines?