I've noticed.

In Mario Kart, something I've noticed is that since MK Wii, Rainbow Road has had space-themed objects the whole time or at least somewhere in the race. In MK Double Dash!! we have a city that's below the racers, but that's not space-related. Here's what I've seen. At the beginning of MKWii's Rainbow Road, there are moon rocks. In MK7's Rainbow Road, we raced through planets and the moon. IN MK8's Rainbow Road, it features modern-looking spaceships, one of which is designed like a Blooper. So that's what I noticed.