I know i'm going to sound really anal now but the Nazgul in Dol Guldur was called Khamul. Also called the Black Easterling. He was second in command among the Nazgul after the Witch King of Angmar.
The Witch King was based at Minas Morgul and Khamul at Dol Guldur.
As for the hobbit movie and a prequel:
If John Rhys Davies was going to be in it, why not as Beorn?Then again Brian Blessed would be good in that role too.
Beorn was my favorite character in The Hobbit. Sir Ian McKellen as Gandalf of course and Andy Serkis as Gollum.
I can see Legolas making a cameo appearance in The Hobbit because he was a Mirkwood Elf.
I think that most of the cast should be fairly unknown. I only knew of a handful of LOTR actors before the movies came out and The Hobbit, like LOTR should be an ensemble piece and not a vehicle for a big name.
I'd be upset if Peter Jackson doesn't make the movie because I'd worry that without him and WETA, it wouldn't be done so well.
Now that we have the LOTR trilogy to enjoy, we can see Middle Earth on film and I for one don't want to see someone else's interpretation. The books are connected and the movies should be too. One thing I loved about LOTR was that the movies followed Tolkiens descriptions of the locations so well. So my mental image of the locations of Middle Earth weren't damaged by the film version. For example Minas Tirith was just as I had imagined it to be, as was Mordor. The only place that didn't fit quite with me was Lothlorien.
I'm unsure as to what they could do about a second movie. The Silmarillion is too complex to film. Unfinished Tales is very fragmented on paper and again too complicated to shoot.
I'd be concerned if someone decided to write a brand new story set between The Hobbit and LOTR unless it was a writer with a passion for Tolkiens books and a thorough understanding of Middle Earth and its lore and history.