Peter R said: "On that topic, are there any existing proposals detailing a canonical ordering of the UTXO set and a scheme to calculate the root hash?"

I created such here: "A Commitment-suitable UTXO set "Balances" file data structure": https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-March/013692.html

In short it periodically makes snapshots on the state of the UTXO N blocks ago, where N = the snapshot period. UTXOs are ordered by TXID. I've also implemented it in C and tested making them.

gmaxwell says the utxo data format will change and I have other recommended changes to the chainstate database in order to make this more efficient. He pointed me to another similar solution... and suggested this would be done later after SegWit and after the UTXO data format was changed in the chainstate database.

Cheers,
Praxeology Guy