I'm more interested in the possibility of replacing these two in the prequels with more interesting characters, for instance, a certain Captain Tarkin.

Honestly, only one of these characters actually speaks! And when he does it's not to say anything particularly interesting - "Order, we shall have order" and "If only Senator Amidala were here" are the only 2 lines I remember. And both lines are spoken with the same flat boring delivery as everyone else seems to have in the prequels.

You both seem to want to insert them to unify the two trilogies (I'm guessing?). I'd rather do it the other way around. There's nothing even vaguely interesting about them.