I'm hoping we're going to get some fundamental physics questions answered. Why can't we unite gravity with other fundamental forces? String theory? True/false? Also, on the high end of the scale: dark matter? What is it? Dark energy? Is the universe really expanding at an accelerating rate? Why? What's the end game there?
Yes, for humans (their robots, that is) to go for a swim in the extra-terrestrial oceans would be a highlight. But, discovery of extra-solar life would definitely be the biggest discovery ever, and also one to fear the most, because the potential consequences would be the greatest, potentially existential.