bitcoin is designed to be DECENTRALIZED, it is a feature!
Therefor it does not need a single point of weekness. your plan is to introduce a SPOW into the bitcoin system, which is BAD. if we did that there would be no need for a blockchain at all, as the central authority could just validate every transaction, like creditcard companies does right now.
Bingo. Funny how people run around in circles because they never take the time to think (or just read Satoshi' paper) WHY Bitcoin does what it does.
They know WHAT Bitcoin does and HOW that is inefficient and try to run around thinking of solutions without asking them WHY Bitcoin does what it does.
The blockchain is based on VOTING. However in an anonymous network you can't limit someone to one person is one vote so Bitcoin doesn't try. Instead it makes it one hash = one vote. Thus the identity of the person producing the hash isn't necessary.