It could definitely be a final reward challenge for a car or an advantage, and I would prefer that over an obstacle course being the final immunity challenge. Since the immunity necklace hasn't been seen in any of the shots yet, it could still be a reward, and they could do an endurance challenge for the final immunity challenge.

And if it is a final 3, then if there's only one challenge, then there's gonna have to be a lot of filler in the rest of the 2 hours.