I feel like malicious downvoting should be defined beforehand. Otherwise we wont't be able to react quickly enough.
Imagine a whale going on an absolute spam/selfvote bender the second this proposal gets accepted.
It would either take weeks for us to find consensus on malicous behavior or we would basically start downvoting him right away, taking away any credibility from the first proposal.
Malicious downvoting has already been defined here as any downvote apart from [1] plagiarism [2] what Community-consensus has agreed is bad behavior.
To move forward we should come up with how the community would reach consensus on what is bad behavior. That can be really hard/impossible to do, so I'm thinking it would be best if we treat each case individually, maybe everytime someone has a reason to downvote another they bring it up to seek consensus and once consensus is reached the downvote can be made with the agreed weight.