As long as I was reading your post Bingo, the same idea you posted came to mind!

You could make Coruscant be the target of the new DS, so we all would know what its destruction would imply, but with no need of including brand new scenes to the movie. The Rebels would be desperate to save the old republic's capital.

However, you would still have to find a reason for the Empire to destroy its core. And what's more, the idea of a planet in danger of being blown up would make ROTJ even more similar to ANH (Yavin)