More levels like the Cliff would be awesome because it's exciting to watch the car tumble endlessly until it reaches the bottom. Maybe a quarry, a large mountain, or volcano would be exciting level additions. Replay features would be awesome too because watching certain crashes over and over again can be fun, especially if something crazy happens. More cars as well, something like a bus, racecar, or something else. Test dummies inside the car would be interesting.
there is a crash dummy stig but its only for some cars there are mods for cars that make race cars and replay seems like its a good idea but i think its a long way off
More cars and maps, doesn't even need suggesting, they are on the way. Test dummies. Suggested umpteen billion times. Confirmed not happening as official content even. Replay, actually a recent experimental patch had a replay button (non functional), next patch removed it again, I think its on its way though.