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.
This is actually a great idea. The way Tarkin was just forgotten about in ROTS was a real missed opportunity - and it's one of many things that make Lucas's point about the episodes being watched in sequence moot. I.e. we're not meant to ask "who's this geezer we haven't seen before" at the end of ROTS. If your idea gets fan-fixed we won't have to.