I think adding more obstacles alone might not improve replayability that much if they all just reduce the number of ducks. From the player's perspective, they'd mostly feel like different sprites doing the same thing.
It might be more interesting if obstacles had different behaviors. For example, red cars could drive recklessly, while cars of another color might slow down or yield to the ducks. That way, players would have to react differently instead of just avoiding everything in the same way.