Using proof-of-stake for upvotes is a very dangerous game that allows for very centralistic manipulation of the rewards. I am not talking about the current situation flag situation but about a future danger from the establishment or big corporations to censor unwanted content.
Instead it should be a smarter system where other factors play a role. I am thinking reputation but that is also to easy to manipulate so more like a meta-algorithm that takes many factors into consideration.
I have suggested this very idea - rolling reputation in as a coefficient, or even creating a separate, second reputation that would better handle this task. Unfortunately, all those options make an already overly complex system even moreso.