Here is a small sample of what I just started working on. I am still looking for the right amount of details and style, so lot of work waiting for me, but it's really fun!
UPDATE 1 (24/4)
Here are some new sketches (and source photos) of Paris Metro stations. I have focused on entrances with it's beautiful signs. There are big differences between them (from huge decorated signs from art nouveau period to simple and minimalistic contemporary ones), so it will be good challenge to keep consistency across the whole project. I am planning to make small series of simple badges (probably just with the signs) and maybe make some small illustrations of entrances, more complex with buildings, trees, cars, people etc.
I am still not decided which theme to choose, but there's a big chance it will be something about France. French wine regions and Paris Metro stations (or more Paris places) are favourites at the moment. But I really like lighthouses, not only the french ones, so I will sketch all of these three themes, maybe try to work with all of them on the computer hope the right idea will appear itself.
Here is just really quick sketches of some lighthouses, french regions and super-quick sketches of few Paris places and trees.
Hope I will move it forward soon and looking forward for Your feedback. Thanks!