While this doesn't address the wallet/keypair issue, one way to more clearly define the role/meaning of the blockchain would be for it to include actual statements of fact or performative utterances.

I don't know what the actual data structure of the blockchain looks like - but whatever it looks like today, it could be modified to include something like:

"I, the holder of private key , with respect to the 5.5 BTC I received in prior block , assert that I am the legal owner thereof, and I further assign all right, title, and interest in those funds as follows: 2.5 BTC to the holder of private key and 3.0 BTC to the holder of private key ."

.. the main advantage I see to that would be that it could potentially turn misuse of someone's BTC's into fraud, given the explicit assertion of ownership, which if false, might constitute fraud. (Fraud has a complicated legal definition which is different from state to state, so I'm using that term loosely here.)

On the other hand, I recognize that not everyone wants to reach that result; but this strikes me as one approach to turning the blockchain into something recognizable to legal system(s).