I agree with your proposal about requiring posting to enable voting. There has been more than a few instances of non-posting high SP accounts flagging, and I have felt from very early on this was not beneficial.

My version of this proposal goes a step further and has a decay rate for reputation, which then enforces continued engagement to have a high rep, and added to the current system, plus your idea of requiring activity to be able to vote, would result in a more careful use of SP and a more clear and effective application of the reputation score to reward contributors and punish those who take away from the platform only.

I have also suggested that witnesses be more constricted in regards to their engagement, that is, as well as votes, the reputation of the witness account factors against their position in the witness leaderboard. There is some objections to that in that sometimes witnesses are too busy doing witnessy things to make posts, but I think if you take your idea, of making 'active account' a prerequisite, you can use this decay curve to enforce it. The decay curve would then also have a decay rate that accelerates over time, after 1 week, maybe you lose 1%, the next, maybe 4%, maybe 3 weeks, 10%, at 4 weeks you lose 25% of the effect of your votes and at 5 weeks your effect is down to 50%.

This would completely stop the use of big stake to censor users because they would have to subject themselves also to the judgement of others by posting.