I think it would be cool if upvoting the same people within a certain span of time would result in less rewards, in order to encourage spreading out your votes.

I think that's a great idea. There needs to be algorithms that incentivize certain behaviors.

It would be cool if Steem did that, so that it was more difficult to have 100 accounts.

I think people with tribe alts might not be too happy with that, but it is what it is.