I added a SHOULD for signing TransactionType.
I have added some text about what ORIGIN and CALLER mean going forward. For TransactionType 0 they are fully backward compatible and the change is invisible to contracts. However, for all other transaction types, the value of both ORIGIN and CALLER will have a transaction-dependent meaning. For ORIGIN, I feel like the risks are pretty low. However, I am concerned that for CALLER the risks in this change are a bit more significant.
Do people think that we are OK to redefine CALLER for future transaction types? We could allow its contents to be determined per transaction type, but require that it always be an address (never some other data) so that existing contracts won’t choke on receiving a non-address CALLER.