If the Blockchain is to be handled by miners or other limited users, then, Bitcoin becomes centralised and open to attack as any centralised entity.  How about Clients only acting on 3 first characters worth of the Blockchain, randomly. IE, My client will only process transactions that begin with 1rP........ If this were hidden from users and done randomly, there'd be no way for anyone to know who was veryfying what portions of the Blockchain  and the blockchain would then itself become decentralised.     So, the idea is, have each client only download transactions that begin  XYZ and will only verify transactions that begin XYZ. It only means that when you send Bitcoin, the transaction sends out a verification request......ahhh, but how does it know where to send the request....It would have to trawl the entire net to find a node that will verify it's XYZ transaction, and so would slow down verification.  And so Ideally, Any node should be able to verify a transaction, but it can't, because it doesn't have the entire blockchain.   So I think I've just painted myself into a corner. What about then, if when you boot up the client it actively seeks out only those nodes which will deal with and verify a XYZ transaction. It might mean a sync delay initially while it finds enough XYZ nodes to verify your transaction but once it's found them, anyway, My thoughts on the Blockchain size problem.   A distributed Blockchain, I'm sure that's what they are working on anyway since it's obvious so I've just wasted 5 minutes of my (and your ) life, sorry about that