[shallowwater]

The only problem with your derivative works proposal is that it won't work. Fans will make fanwork of things they are fans of, regardless of the legality involved. Akin to the culture of digital sharing, that horse has already left the barn, it is pointless to try and close the door now. If you wanted to ban only commercial derivative works for a longer period, that might work, but non-commercial fanwork isn't going anywhere.

[repsilat]

There are two arguments to be made here. The first you'll probably agree more with:

I think the answer to that is that nobody cares about kids writing fan-fiction as long as they're not selling it. If copyright exists to "promote the progress of science and useful arts", I don't think you'll find an artist deciding not to write or publish their work because non-commercial derivative works might be made. Exclusive rights to commercial derivative works, on the other hand, might convince people to create new things.

The second point is that whether or not enforcement can be completely effective isn't really relevant. As long as it can be somewhat effective, and as long as people believe the law is just, people will believe that the law is worth keeping. Governments tend to believe that "the culture of digial sharing" is a moral evil, and that not going after it would be to abdicate their duty. "It's too hard to police" isn't a good enough excuse for pot or gambling or seatbelts, why should it be good enough for fan-fiction?

[shallowwater]

I do agree with your first point, and it would basically be a continuation of the status quo, which seems like a pretty reasonable compromise between allowing people to profit from their work without stifling the creativity of others and criminalizing harmless behavior that is going to keep happening anyway.

My rebuttal to your second point is to acknowledge that you have part of a point, but also to draw a comparison between the futility of policing digital sharing and the futility of policing fanwork. Since we're (sortof) making a 'in a perfect world' wishlist, I think we can do better than fall into the same traps that current law/lawmakers/our assorted predecessors fell into, criminalizing behavior that is entirely or mostly harmless and where enforcement has minimal benefits.

With your other examples, I think that there is a case to be made on both sides with regards to the dangers vs the benefits from enforcement. But with those cases, I can more clearly see the arguments that enforcement has benefits that are worth the difficulty (depending on how damaging you think pot, gambling, and car accidents are). So I could see where a person would argue that noncommercial fanwork is severely enough damaging to the creator/rights holder to make enforcement worth it, but I personally don't see it as such and would have a very hard time believing such arguments without strong proof. Car accidents where someone is not wearing a seatbelt, on the other hand, has pretty demonstrable damages.

(I'm not really going to get into pot or gambling, because I think ath this point, the arguments for and against pot criminalization are well-established, and starting to weight every-so-slightly toward decriminalization, and I personally find gambling distasteful and boring and don't have the energy to read up on the damages associated with gambling or the benefits associated with enforcement of anti-gambling laws.)