In my opinion, the plagiarism problem is a symptom of one of the greatest flaws of Steem/Steemit: that the content (or at least, the text) is posted on the blockchain. (At least that was told to me at Bitcointalk)

I would recommend that Steem should limit itself to the monetization/tipping/voting part - which is a great improvement over similar systems than Flattr and the Bitcoin-based tip systems. But the blockchain transactions should only contain links to the posts - full text would be saved on Steemit and other websites. This way, we had a network of "steem-based websites" where every webmaster would be responsible for the content, like it occurs now with forums and other social media.

In this scenario, plagiarism would not be incentived because it would be simply marked as duplicate content by search engines and Steem websites full of plagiarism could be even deleted from Google's index because of poor content quality. Nobody would want to visit such a site. All the contrary: steem authors would have a strong incentive to post only unique, quality content. And: If one webmaster of a steem-using site doesn't care about plagiarism, it would not affect the hard-working quality-content authors.

The disadvantage is that it would be more complicated to run a centralized "social network service" like Steemit, and that some "Steem based sites" could go offline. But in my opinion it would be a more sustainable model for a blockchain-based content monetization system like Steem.