Thinking about it a bit more, and pairing it with Adding date complete to city and Badges for milestones and the fact that a city update can change the city in a way that changes your percentage from 100% to something lower …
It’s been a while since the badge/achievement idea went up, but I am still actively thinking about it & I expect one of the first badges I release will be a “Completed [city name]” badge. This way you can earn it, the city can change which decreases your percentage, but you retain the fact that you did at one time complete it.
This could also help fix up a bug where you complete a city … have it connected to Twitter so you tweet about it … the city gets updated in a way that adds nodes … your account is reprocessed and those nodes are already completed by past acitivties … which marks the city as complete again … and sends off the tweet again. 
Long-term/future additions could include a first/second/third place that can shuffle around if someone else provably finished it before the current first-place-holder.