A fairly radical idea that I just had: When Palps sends the order for 'Order 66', let's not have it received by "Commander Cody" or any clone on Utapau for that matter. Instead have it received by a suited officer on Coruscant (I've never liked the fact that SOME clones can advance to "commander" status - they're all identical and disposable!)

Obviously this would require some refilming of a costumed actor in front of a green screen, but as far as the ideas here go, it's a pretty simple one. Get an amateur actor who looks a bit like Peter Cushing and it can even be 'Commander' Tarkin's career defining move, and why he gained such favour with Palpatine and was able to talk to Vader the way he does - he was loyal to Palps and had this 'Order 66' in place long before Anakin turned.