Hmm....honestly, I think sewers are quite significant in terms of bringing health benefits in cities. My suggestion is to make them more expensive to build, to justify a bigger side benefit. Some ideas: 1 slot for engineer: sewers are intricate infrastructure systems and have civil, mechanical, electrical and environmental engineers on staff to design, run, monitor and upgrade facilities, along with other skilled technicians involved in the whole process. Keep in mind that proper sewers are not just a network of pipes and tunnels to move wastewater away, there's also water treatment plants, pumps and so on. Have some stronger effect than merely extra health points (maybe stopping the plague altogether?) when confronting the plague events. Usefulness might be limited due to their appearance in the industrial era, when most plague events would have already happened. It would, however, higher difficulty levels in which technology progresses much faster due to the AI bonuses outpacting the human player's default ability to conduct research.