If say Shannen and the producers did manage to resolve there differences, say a year after she left for instance, this is how I'd see Prue returning throughout the remander of the series.
sympathy for the demon - Prue and Paige meet at the end of the episode after Paige does a spell Prue did.
centennial charmed - little tribute scene to prue where Piper, Paige and Leo visit Prue's graveston.
cat house - Prue flashbacks of course
season 6 -
crime and witch demoners - prue flashbacks of course
season 7 -
something wicca this way goes? - Prue as a ghost helps her sisters, and we actually see Prue as a ghost closing the door at the end.
season 8 -
forever charmed - Prue from the past helps in the final battle.
Cant think of any others? What episodes would you put Prue in, if things did work out for her appearing and flashbacks and photos used.