Shanghai and Washington DC would both work well - Y shaped river layouts.
I'd like Chicago even though the lakefront wouldn't really play into the game very much. If you can manipulate spawn locations to keep the squares along the lake, though, maybe you could generate some fun linear/radial maps. And there is a river to contend with as well.
Seattle would be excellent - peninsula, lakes, etc.
Tianjin has a really fun river shape, but it'd just be another river map.
Finally, cities on straits could be interesting especially if tunnels couldn't be of infinite length (i.e. you had to tunnel only at certain points where the strait is narrower before it opens back out). Candidates might include Istanbul or Detroit + Windsor.