So my question is whether anyone can see ways in which this introduces
redundant flexibility, or misses obvious use cases?
Hopefully my comment is on-topic for this thread:
Given that we want to move away from OP_CODESEPARATOR, because each call to this operation effectively takes O(script-size) time, we need a replacement for the functionality it currently provides. While perhaps the original motivation for OP_CODESEPARTOR is surrounded in mystery, it currently can be used (or perhaps abused) for the task of creating signature that covers, not only which input is being signed, but which specific branch within that input Script code is being signed for.
For example, one can place an OP_CODESEPARATOR within each branch of an IF block, or by placing an OP_CODESEPARATOR before each OP_CHECKSIG operation. By doing so, signatures created for one clause cannot be used as signatures for another clause. Since different clauses in Bitcoin Script may be enforcing different conditions (such as different time-locks, hash-locks, etc), it is useful to be able to sign in such a way that your signature is only valid when the conditions for a particular branch are satisfied. In complex Scripts, it may not be practical or possible to use different public keys for every different clause. (In practice, you will be able to get away with fewer OP_CODESEPARATORS than one in every IF block).
One suggestion I heard (I think I heard it from Pieter) to achieve the above is to add an internal counter that increments on every control flow operator, OP_IF, OP_NOTIF, OP_ELSE, OP_ENDIF, and have the signature cover the value of this counter. Equivalently we divide every Bitcoin Script program into blocks deliminated by these control flow operator and have the signature cover the index of the block that the OP_CHECKSIG occurs within. More specifically, we will want a SigHash flag to enables/disable the signature covering this counter.
There are many different ways one might go about replacing the remaining useful behaviour of OP_CODESEPARATOR than the one I gave above. I would be happy with any solution.