I believe your assumptions are flawed:
- People will structure their questions to a much higher quality.
Given that the majority of 'bad' questions, in my experience, are posted by new users (rep < 200), and often without appropriate formatting, I suspect they ask the questions without regard to the social mores of the site, and ask simply to get an answer.
- Those "not playing fair" with very low accept rates will actually notice the disrepute they're attracting. Whether or not this is the intended use of down voting, it exists and is a form of community punishment.
Those that don't play fair implicitly aren't that interested in the social aspect of the SE network. I don't think they'd really care about the increased punishment.
- Unnecessary or repeated duplicates (users will often repeat a question until they start getting some answers) can attract penalties.
Not sure how to address this one, sadly. It's a behaviour I've observed, but...no idea what use sanctions would be against this.
The problem, I think, is that you're starting with the wrong assumption: that a downvote is a punishment. It's not, it's simply a measure of the community's measure of how 'unclear' or 'not useful.'
In short, I don't think that increasing the rep-hit for a down-vote would serve a useful purpose. I certainly don't think it would serve, or is intended to serve, as a punitive measure. If a question is 'not useful' or 'unclear' (as the down-vote is intended to measure) then the question will be either unanswered, or down-voted from the front page and so less-exposed to the community. Repeatedly asking the same question is likely to garner several comments and flags prompting the >3k users and mods to close/delete the questions. Which is as much 'punishment' as I think is deserved, really.