Shut the downvotes off and focus on the positive curation?
This is the only option that I would not consider at all.
I can't really say what the best thing to do is besides flagging at all. I don't really think limiting downvoting decisions solely to "vetted" users is a good solution, although leaving it to hivewatchers (= steemcleaners) would probably be pretty good. They proved themselves very reliable over a long period of time with only very few cases where they didn't quite behave accordingly to the situation (in my opinion)
I could also see a solution where all delegators are able to "nominate" for flagging but each flagged is looked over shortly by a more experienced community member.
I'm glad I'm not in your position to decide this at the moment ;)
Greetings
Schwein