Courts
What are the unamendable portions of the Constitution? Why did the Framers make them unamendable? Answer to the first--see Article V, last two clauses.
Hint: In other words, why is federalism a central American political principle (and what is its relationship to majoritarianism) and was the founding racist? I doubt that any senator of either party has any interest in posing the questions, but I have to have hope.
For the other questions I would have had them ask of nominee Sotomayor, see here.
I hope they also bring up Kelo and the nature and purpose of eminent domain.