Anonymous downvotes would be interesting, but could also lead to frustration and even more abuse. I think the PoB system as it is today is not that bad after all, especially after the 50/50 curation change, things got drastically better. Any other attempt like free upvotes to counter a downvote could unfortunately also be misused, but I like the idea to somewhere appeal a downvote, making the case visible (does not need to be a proposal but could also be a special page), so that other users can judge and either up or downvote the post.