Realistically (no "Flying Cars for All!") , you are granted the power to change one law (existing or not yet existing), what would you change?

I would have to agree with that. It would have the most positive impact on everything.

That's a good one.

Given how the Trump Presidency has unfolded in the U.S. I think it might be advantageous for Congress to have have the same Q&A system that the Brits. have once a week.

In the UK Parliament the Prime Minister has to answer questions put forward by other Parliamentarians once a week in a system known as Prime Ministers Questions.
I'd like to see that extended to maybe two half-hour periods a week.

A similar system would be advantageous in the U.S. The Executive branch of government has clawed so much power into the highest office that the supposed system of 'Checks and balances' no longer functions as it should.
If the President were put-on-the-spot and regularly questioned by other elected officials, citizens and others would have a far clearer idea of what the intentions for certain actions are.

It's also a clear transparency that those 'Checks and balances' are at least being seen to be there.

Massive campaign finance reform.

No PAC's. No 501(c)4's. No corporate contributions. No foreign contributions.

Only individual contributions capped at 10k$ per person. Donations only from people eligible to vote.

Contributions to parties similarly limited.

Death penalty for pedophiles.