Well, Timothy Zahn first came up with the name Coruscant for Heir to the Empire. Lucas just liked it enough to use it in the prequels. In earlier drafts for Jedi, the Imperial capital was referred to as Had Abaddon.
As for The Journal of the Whills, it was a literary device used in early drafts to make it sound like Star Wars was a small story told from a huge history of the galaxy. It was eventually superceded by the "A Long Time Ago in a Galaxy Far, Far Away..." opening. However, fans picked up on the name (most likely from its mention in the novelization of Star Wars where the forward is said to come from The Journal of the Whills) and imagined it to be an actual chronicle of Star Wars stories that George Lucas had written. Later on, as a bit of self-referential humor, George compiled all of his notes in a binder that he jokingly called The Journal of the Whills.