Three suggestions that came to mind after playing this game for a while (skipping over performance issues, as they seemed to be from the Google street view API and not the app): local storage to save any progress when accidentally leaving the page, specific region gameplay and custom mapping for challenging friends.
By "specific region gameplay" I mean allowing the player to crop a certain region on the map and let them play within that area. For instance, America could be cropped and the player could then be tested at his/her ability to spot the different characteristics of each state. This basically just adds a certain ease to the gameplay instead of being placed into a secluded forest in Asia.
And custom mapping is basically how it sounds: allow the player to place their own markers and share the link with friends. Though since the "sharing link" is currently just a base64'd JSON string, I'm sure customized links could be created with a little manual effort if needed.