"I don't really care what their goal would be, as long as it would be a goal that they could work towards..."
and that link shows the basic four orienting science questions (right panel in blue text) - how did life begin, how did the solar system evolve, how did the planets originate, etc.
Those are the top-level questions that the National Academies panel doing the Decadal Survey arrived at and that motivates the portfolio of planetary missions that are planned. I would have linked to the Decadal Survey (overview at https://en.wikipedia.org/wiki/Planetary_Science_Decadal_Surv...), but the report itself is a very long pdf - and this gets into the strategy in detail.
For instance, the next Mars mission (Mars 2020) is set up to address the "life" question, and the "how did the planet [Mars] evolve" question, etc.
Think about SpaceX. What they've done should have been impossible. A private company operating on a shoestring budget revolutionized space in 15 years? That doesn't mean Musk is a genius, it means that our current space system is simply incompetent. There are certainly many reasons for this, including congress using NASA as a jobs program, but I think a lack of direction is also a major reason. 'Discovering how life began and evolved on Earth' as a mission statement is like a philsopher stating his goal is to 'discover the meaning of existence.' It's a goal that's so impossible and so broad that anything can be justified in its name.
'Easier' goals are in many ways much harder since you need to remain focused. And when NASA has focus they, at least in the past, have shown themselves capable of amazing things. Going from having never even put a man in orbit, to putting a man on the moon in a decade? Think about that, if you even can. In today's times that should seem completely unbelievable. And we should be capable of exponentially more now a days. And that is where we should set our sights. The point I'm getting at is that if you aim low, that's always where you're going to hit. Aim high and you might miss, but at least you're pointed in the right direction.
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As one brief aside, those mission statements immediately made me think of Oumuamua [2] -- that cylindrical weird interstellar object (it's exact nature is, and will likely remain unknown - the name translates to 'first scout', as the imagination might imagine) currently looping around the sun on its pass through and on its way out of our solar system. Being able to get ships and ideally humans onto that thing would be groundbreaking and actually go a long ways towards pushing those mission statements, abstract though they may be, forward. But our space program and technology is so backwards and dated that we're instead left watching jaws agape as arguably the single most relevant tracked celestial object lackadaisically (...as lackadaisical as 30km/s can be) laps us on its way on out of our solar system. It's again absurd that the primary interest in trying to perform an intercept with this object has come from the private industry. But hey, at least we'll know the wind speed in Uranus...
[1] - https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201700...
You mention the ambiguity of the animating goals. Part of this is just linguistic: of course you want to push down these top-level goals to more granular, 0/1 goals ("was there an ancient lake at this site on Mars?" Answer: yes). That's what the NAS report does. Now that we know how abundant water was on ancient Mars, we'd like to return samples to see if there was life. That's the next couple of missions.
You raise a worthwhile point, I think, about immediately-graspable 0/1 goals, like "man on moon". I see your point - but I'm really only here to talk about the unmanned program because my limited expertise covers part of that area.
You like SpaceX. So do I - two good friends left my lab to work there. But their achievement ("revolutionized space") is not unique. The little Mars pathfinder had about that much history. There are plenty of other examples over the same time period, like exoplanet discovery (as you noted), cosmology, and a host of Earth science stuff.
So to make this more clear, one reason I think what SpaceX has already achieved and especially what they plan to achieve is so revolutionary is because of how it would impact other programs. Like you mention one of NASA's big goals is a sample return mission. And this is going to be highly complex and highly expensive. The OSIRIS-REx mission mission is a billion dollar, 7 year mission to get 0.1-2kg of debris from an asteroid that comes within 0.0002AU of Earth with a 6 year period. That's really not reasonable. These missions would be trivialized with technologies such as the BFR. Such technology would also enable vastly greater scope and scale of these sort of missions. A Uranus orbiter will provide some science that might have some value, but it's not really advancing anything. The chance of revolutionary discovery is practically 0, and it will be unlikely to have a meaningful effect on future missions or projects.
Basically it feels quite odd that we're using technology that is comparable in both price and capability to what we were using the 70s. It's quite peculiar, and I think this has been the major hold up in us achieving much more in space. Imagine for a minute that it's 1969. We've just seen live footage of men walking and driving rovers on the moon. And I tell you that in 50 years NASA's vision for a flagship mission will be sending a probe to Uranus. And no man would ever leave low earth orbit again after 1972. Wouldn't you say something has gone very wrong? And this wrongness continues to persist, and I think it is because of this 'Well I have $x. What should I spend it all on?' as opposed to aiming for directed evolution and progress.
