First off, there is background scenery.
Background scenery can exist in multiple layers which scroll in a parallaxy fashion, to give a bit of depth. Though not seen in this shot, it looks really good as you go up and down hills.
Also, it's possible to flag particular elements of scenery so they're scaled up as you drive along the track. Outrunners used this in a few places to have some giant object that you get closer as you drive along, like St. Basil's Cathedral in the Moscow course.
Finally, I added a few more features to the roadside sprites. They now collide more accurately with vehicles, and the collision can have an arbitrary effect besides just being considered a solid obstacle -- slowing down the player when driving through those puddles, for example. The precise collision information would also let me do sprites that are only partially hazardous, such as arches over the track or distant clouds.
I'm hard pressed to think of any other low-level features present in the Outrun series that are missing from this engine (other than "player control that isn't totally broken and half-assed.") So... that's good, I guess. I may work on this a little more tonight -- hopefully in the context of drawing some decent sprites! -- but in the meantime let's call it:
DAYS COMPLETE: 5/7