These are really good suggestions. The witnesses and stakeholders have actually talked about them quite a bit.

The problem with the diminishing returns and 4 post limit proposals is they are only a deterrent. It may seem difficult to a ‘normal user’ to setup 20 or 100 accounts, but for the people who are abusing the rewards pool for hundreds or even thousands of dollars - they will put in the two hours of effort to create a bunch of accounts to get around the rules. The users who stay “in bounds” will be at a disadvantage, and the net result is the true abusers would be able to extract more than they could today.

The alternate rewards curve would help lower the amount of rewards that go to abusers, but only to an extent. The ones with enough SP (including leased delegated SP) to get over the initial ‘hump’ would still be able to reward themselves a lot through self voting.

UserAuthority is an awesome idea, but unfortunately it will not fly for STEEM rewards. The reason is that it breaks the contact with all of the existing SP holders that bought SP under the premise that more SP meant more influence over rewards. It could very well be something that could be implemented with SMTs though.