I didn't find any other thread concerning this issue, so I am leaving my suggestions to general track types here.
I would also love city/starship type tracks !!!!
Jungle tracks with some hardly visible turns
Stadium tracks with funny obstacles like loops jumps and wild, narrow sections. Yeah
Beach scenario with some tunnels under water. Perhaps a shortcut which a certain amount of speed to cross 100 m on the water surface.
Vulcano scenario: I just want to see glowing lava flowing next to the mostly black track. Would make some epic views and pictures.
Perhaps it is reasonable to gather the track type suggestions in this thread and start another poll to get some sort of priority.
Let the comunity help you. I know it is some effort to create a map editor, but it might pay off. I am sure the community might came up with some nice tracks. To avoid a confusing amount of semi-good tracks, like in some other games, you could introduce something like official tracks, where you add some really good community tracks, which are selected by the development team to ensure some level of quality. In this context you could arrange map creation contests.
Same can be applied for new cars. To avoid overpowered cars, the creator might be distribute a fixed amount of points to the car characteristics.