When designing a circuit, it is often a good idea to create a theme and a focal point.
If Pit Stops are a feature that will be a major part of the game play then the pits complex probably needs to be close to hand and accessible as giving a clear line of sight from the drivers viewpoint.
Bridges and tunnels are a popular feature but make sure that cars can be retrieved from these areas.
A long straight to test the cars maximum speed or acceleration is often a feature of a circuit. Make sure, though, that the area at the end of the straight is accessible for the retrieval of de-slotted cars. Also make sure this area has some kind of soft barrier to avoid damaging the car unnecessarily.
