If roles between producer and sequencer are split; why not give block producers a double role? 1) Sequencer: ordering the transactions from the previous block based on hash ordering (with some data from the current block. E.g. pub key); 2) Selector: selecting transactions for the next block.
To the selector the final ordering of transactions appears to be unpredictable and random. The sequencer has only the possibility to drop the last transactions if they exceed block gas and has no possibility to insert/replace orders. The sequencer can only execute a predefined hash order and does therefore not have an arbitrary impact on the order.
Please let me know if such a method has already been proposed (and rejected).