[Chappi]

What is the actual problem you have with creating city maps? As a generic tip I suggest you to play a bit on the multiplayer maps so you can check the overall design of city maps.
I'd say keep the buildings limited so it's not a concrete jungle, make sure to add details such as shrubbery and sidewalks. For extra touch you can fire a few shells on the buildings to give them a destroyed look and place a few car/tram wrecks. Also you should take in mind that tanks shouldn't be able to navigate easily in the city and some streets should even be too tight for them making infantry the only option. Create a few "danger" zones where the enemy will be entrenched, so player will have to think how to get to him. Additionaly its a nice idea to let player achieve the task from multiple ways but that's a general maping thing

As for the wwI theme - I can't really help you. It really depends if the mod team wants to keep everything history accurate. If yes, then you should read a bit about the battle you want to portray in the map editor. Google the name of the city and try finding some aerial views of it(if there are any from wwI). If they are not so history accurate, then you should fantasize and let your imagination do the work. Just keep in mind it's a wwI map

Here are a few reference photos for you.