Here's my variation on the evolving blockchain security model.
Rather than one notary, any key with more than 1% votes becomes a node on the unique notary list. A new block requires signatures from ALL nodes on the UNL. Vote output tx's have a revocation certificate so you don't need to get coins out of cold storage to pull your support for a node. If less than three nodes have more than 1% votes then the top 3 get on the UNL - this should never happen.
So there is a theoretical max of 100 nodes but realistically you will have nodes run by each major exchange (motivated to protect themselves against double spend) and a few hardcore crypto ancap activists.
The idea is that the presence of even ONE non-collusive notary is enough to prevent double-spend attacks.
The tradeoff is that even one node can stall the whole network. You can produce a block without a node's signature if that block includes transactions that remove enough votes for that node. I'm not sure what exactly to do if a node stops signing and then nobody takes their votes off of that node.