If a theme is needed for the PT, couldn't that theme (or a background theme) be more emphasis on the formation of the rebellion.
Knowing Adywan plan to shoot new footage, even trying to add one additional scene between two established characters would be difficult. For example between Obi Wan and Anakin you would need two actors with a striking similarity in both looks and voices. Also costumes would need to be very close replicas. But even then these need to be good actors otherwise it would just be more weak performances to add to the PT. Obviously one of both of these characters could be recast, but then that presents the problem of compositing the new actors seemless in place of the old actors.
My suggestion would be to recast a very small number of characters that have very little screentime, such as Tarkin, Bail Organa, and a rebellion leader such as Mon Mothma. Bail would be the biggest change, but it would allow much of the politics in the films to be altered to something more interesting.
An example would be to make the gungans a species that lives on another planet near to Naboo, but not Naboo itself. They can arrive on Naboo in order to help free the Naboo, along with a small allied force from Alderaan lead by the newly recast Bail. The Naboo fighters can beome Alderaan fighters, and the space battle now becomes Naboo being liberated by Alderaan and the gungans.
The reason I would like to see more Tarkin is because it can push Palpatine into the background, helping to preserve any surprise he is evil, by making Tarkin appear to be the one who is pushing for increased military and a stronger, harsher republic. Also by having full control over what footage can be created between these new actors, a scene could be made showing Bail being reprimanded for taking unauthorised military action, and later, the republic ordering the demiliterisation of Alderaan.