Hmm, I wonder if I could make a list.

Journeying to the center of the Earth is not really possible, but what about something similar with our Moon?
Traveling more than a couple miles down on Earth is different than traveling a couple miles down on the Moon. At what point in depth on the Moon would one uniformly get thermal conditions similar to say 5 miles below the Earth's surface. And similar question with Mars, Mercury, or perhaps even Venus.

Nine more questions. Hmm.
I guess knowing more about earth oceans- the heat vents, the pools higher salinity, etc, etc.

Well, then also more about the Earth atmosphere at and above 100,000' to boundary of space.

Seven to go. Is there a liquid water ocean or lakes on the gas giants.

And obviously, is there minable water on the Moon and Mercury?

Why isn't there and was there ever significantly higher mountains on the Moon. If you are talking about a short period of say 10,000 years, what have been the tallest mountains- on Earth, Moon, and elsewhere.

Has the atmospheres of Earth, Moon, Venus, Mars, etc changed quantitatively in a significant amount over the last 4 billion years. Did the Moon have Mars like atmosphere for say 100 or more years. Did Mars ever have less atmosphere than the present. How long has Venus' atmosphere been like it's current atmosphere.

Of course, NEOs and asteroids aren't planets but they certainly connected. So, regarding NEOs and all the asteroids, we know hardly anything. You have separate list about NEO or Kuiper object. But how about what is the biggest rock to hit the Sun in last 4 billion years and did this effect upon the Sun have any significant affect on the planets.

Two more. Would like to know more about lightning and dust on Jupiter.

And more about the oceans of Europa.