Thank you for the reply. I look forward to city congestion, it really worked well in sim city 4 seeing road congestion and I think it's a nice idea. Did you think about different road types? Like dirt, log and cobblestone.
Answering your question though: Some administrative building with a global bonus for really large cities, or one that lowers upkeep by a flat percent (something small, like 10%), or a huge "wonder" type building giving happiness, there are a lot of fun options out there.