I believe core bitcoin developer Peter Todd had a couple of ideas here:  http://redd.it/281ftd

I don't have much to add, except that a crazy thought just occurred to me where we add a PoS layer to the blockchain so that we have two layers of distributed consensus, one PoW and the other PoS.  Then the PoS layer can be used to regulate the PoW layer.  In other words, those of us who hold bitcoins can actively assign or remove trust from mining nodes (which could be pools or individual miners).  The trust could in theory limit the number of blocks they were able to submit on the PoW side.

I admit I haven't "thought this through" very far and it may not be viable at all.  However, the idea popped into my head after reading one of Danny Hamilton's last posts and I thought I should mention it as I haven't seen it discussed before.