Well, here's a list from Hit Fix (the whole article can be found here) that goes into who some of the possibilities (realistic and not) might be:
THE DREAM CHOICES
Best known for: Being one of the greatest filmmakers in history.
Why: It took him quite awhile to commit to direct "War Horse" and his heart may not truly be in it. He's already worked with Jackson as a collaborator on "The Adventures of Tintin: Secret of the Unicorn." The legendary director has also recently waxed about wanting to create an immersive world like James Cameron did in "Avatar." Can you say "The Hobbit" in 3-D?
Available: If he delays the recently announced "War Horse," yes.
Would he do it: There a lot of bumps to get over to make it happen. First off, two years is a long time for Spielberg to commit to and its doubtful he'd spend a year shooting in New Zealand instead wanting to move much of the production to sound stages in LA (not cheap). "War Horse" is also a new DreamWorks film and delaying it would mean waiting over three years for Spielberg to direct a feature for his recently rebooted studio. That would not make investors Disney and Reliance very happy. More importantly, could his ego stand being, effectively, the "second choice" after del Toro departed?
Best known for: "Charlie and the Chocolate Factory," "Edward Scissorhands"
Why: Has virtually become a cinematic "brand" having created a legacy of iconic characters and memorable movie images.
Available: No? He's supposed to be prepping "Dark Shadows" for Warner Bros, but you'd have to guess the studio would let him delay that flick to work on their own "Hobbit."
Would he do it: Burton is hardly one to be meddlesome with scripts, but Jackson would have to give him huge leeway to present his own unique vision of Middle-earth.
Best known for: "Harry Potter and the Prisoner of Azkaban," "Children of Men"
Why: One of the finest filmmakers of his generation who can easily segue from "Y tu mama tambien" to "Harry Potter."
Available: At the moment no. His new sci-fi thriller "Gravity" is about to begin production for Warner Bros. Afterward? Another story.
Would he do it: Considering he is good friends with del Toro he'd have to be intrigued, but the timing seems as though it could be off.
THE SAFE SECOND TIER
Best known for: "Back to the Future," "Forest Gump"
Why: Arguably one of the most creative commercial filmmakers over the last 30 years who just happened to be overshadowed by Spielberg, Scorsese and Cameron.
Available: In theory, yes.
Would he do it: After the disappointing results of "A Christmas Carol" and Disney shutting down their co-venture in motion capture, it may be time to return to the world of live action. And Middle-earth has more than its share of characters who could fit into the motion capture realm.
Best known for: "Pirates of the Caribbean"
Why: He's been able to create a unique cinematic world with the "Pirates" films even if the scripts failed him at times. He's certainly talented.
Available: It appears so. Besides the animated "Rango," none of his projects in development are close to being greenlit.
Would he do it: Unclear. After spending years on the "Pirates" films does he want to dedicate two more shooting and editing two "Hobbit" features?
Best known for: "Shrek," "The Chronicles of Naria: The Lion, the Witch and the Wardrobe"
Why: Extremely imaginative and talented New Zealander who has made a successful transition from animated blockbusters to live action blockbusters. And having made brought the first two "Narnia" books to the big screen he has an acute understanding of the detail that must be given to create an immersive, cinematic world.
Available: Having taken over two years off since "Prince Caspian," yes, he certainly is.
Would he do it: Considering it would allow him to live at home in New Zealand with his family and the respect and admiration he always heaped upon Jackson's "Lord of the Rings" trilogy, it's hard to imagine him turning it down.
Best known for: "Spider-Man"
Why: He really wanted to direct it before del Toro got the gig.
Available: He's reportedly prepping "No Man's Land" after "Spider-Man 4" fell apart, but you have to imagine he'd quickly jump to the "Hobbit" if offered.
Would he do it: Uh, yeah, but after passing over him the first time, would Jackson actually hire him?
THE UNCONVENTIONAL REACHES
Best known for: "Amelie"
Why: Hands down one of the most inventive filmmakers ever.
Would he do it: His one experience with English language films, "Alien Resurrection" was hardly a happy one. Jackson would have to do a lot of convincing.
Best know for: "Atonement"
Why: Incredibly talented filmmaker with a keen and unique visual sense.
Available: After he finishes shooting and editing the thriller "Hanna" (it's currently in production), yes.
Would he do it: Wright is a pretty strange guy for someone whose made his name in period pieces such as "Pride and Prejudice" and "Atonement." Your guess is as good as anyone else, but he'd be an unconventional, but exciting choice.
Best known for: "Slumdog Millionaire," "28 Days Later"
Why: Like Lee, he's been able to jump from different film genres while still keeping his unique perspective on the world. And simply, the man has an eye and a way with actors.
Available: He's in post-production on the thriller "127 days" which should release this November, but after that it appears so.
Would he do it: Boyle has admitted he's not a fan of the lack of freedom massive studio budgets entail, but with a few Oscars in his hands, he might feel it's time.
Best known for: "Wanted"
Why: Groundbreaking visual filmmaker who continues to surprise.
Available: Probably not. He's supposed to direct an adaptation of "Abraham Lincoln: Vampire Hunter," "The Last Witch Hunter" and a somewhat delayed "Wanted" sequel for three different studios. That's a lot of gigs to push back for two years.
Would he do it: Sadly, it probably doesn't matter, the man may just be booked.
Best known for: "Crouching Tiger, Hidden Dragon,""Brokeback Mountain," "Hulk"
Why: The Oscar winner has shown an uncanny ability to succeed at multiple genres throughout his career.
Available: Unclear. Lee's next film, "The Life of Pi," is rumored to be on hold over budget concerns.
Would he do it: Another ego question. Would Lee be able to collaborate with Jackson? Unless he has creative freedom, it might not be worth it to him.
Best known for: "District 9"
Why: Protege of Jackson's who made a spectacular and critically acclaimed debut with "District 9."
Available: Supposedly prepping a secret project for film financier Media Rights Capital. If so, that's a "no."
Would he do it: Even with his friendship with Jackson, very unclear.
Best known for: "The Bourne Supremacy"
Why: He's no stranger to genre having almost directed "Watchmen" and after "Green Zone" he needs a hit and to show he can do something other than his increasingly repetitive hand held aesthetic.
Available: It depends how quickly he can shoot and edit Universal's drama "They Marched Into Sunlight."
Would he do it: Unclear, but his agent will certainly want him to pitch it.
ONLY TO AVOID BRETT RATNER OR McG
Best known for: "Constantine," "I Am Legend"
Why: Criticisms over script choices aside for both his big screen efforts, the man has vision. Plus, he did stellar work on the underappreciated NBC series "Kings."
Available: Not at the moment. He's currently filming the drama "Water for Elephants" for 20th Century Fox.
Would he do it: Unclear.
Best known for "X2," "The Usual Suspects"
Why: He's had his share of success with genre franchise and publicly stated his preference for films with multiple character storylines.
Available: Since "Jack the Giant Killer" is on hold for casting and script reasons, he sort of is.
Would he do it: Are you kidding? Singer has attached himself to more genre projects than any other studio director in town. Whether Jackson wants to deal with the "reformed" infant terrible is another matter.
Best known for: "The Descent"
Why: On a strikingly similar career path to Jackson's before he was able to get "The Lord of the Rings" off the ground. Deja vu?
Would he do it: Would be career suicide to turn it down.
-Who would you pick?