What about adding a slight bonus to answering more highly regarded questions? For example, if I answer a question that has been upvoted +1, and my answer receives an upvote, I should get 11 points instead of 10 (10 for the upvote, +1 for the question upvote). That way higher-upvoted questions would be more attractive to answer, garnering better answers.
I don't think this is a good idea.
I see your point, but similar requests have been made before and not been recieved well. Jeff has stated before that he does not like to add too much complexity to the voting system. The only rep/voting value change I see happening in the future is an increase in the downvote rep loss.
Also, people get confused enough at the current voting system and its features.
Olafur brings up a great new problem also. This is very good example of a future problem that would be introduced with the rep change proposed, which would affect the core goal of the SO sites, which should not be compromised.
Right now, it's easier to gain rep by answering questions on popular topics. These are also the sorts of questions that are likely to get upvoted. I don't think we need to increase the rep gained on popular topics versus unpopular topics.
Nor do I think it would be good for getting questions answered, regardless of topic. People looking for rep wouldn't answer a wide variety of questions, which is generally good for the sites, but would concentrate on the higher-voted questions, leading to more answers than the popular questions can use, and insufficient answers for the less popular questions. Again, popular questions already are likely to be the ones that attract answers, and I don't see any reason to emphasize this.
It also means I can vote directly for my own rep. If I provide a good answer to a question, I'm going to be tempted to upvote the question. I'd rather keep answers and question upvotes entirely separate. Yes, Jeff could allow for that. In fact, Jeff can make the voting system arbitrarily complex, but I don't think that's a real good idea either. Simple systems are more likely correct, easier to understand, and don't result in unintuitive corner cases.
In order to support something like that, I'd like to see clear explanations for sizable benefits we'd expect.
Then people would rather wait for an upvote to a question to get the maximum amount of rep.
That would also be biased against technologies that are unpopular.