Ask any Bat-geek out there, and they'll tell you that Batman has two great nemeses: The Joker and Ra's Al Ghul. These are the only two villains who have ever even come close to destroying not only Batman himself, but his entire world. In many ways, these villains are total opposites of each other. The Joker is sporadic, insane, and does what he does simply because he loves doing it without any method or thought to his madness. Ra's Al Ghul on the other hand, is maniacal, crafting, and does what he does to serve his extremist idealism. While other villains like Poison Ivy, Mr. Freeze, The Riddler, Bane, Scarecrow, Two-Face, etc. can be easily sympathized with, and really only do what they do because of greed or revenge, Ra's Al Ghul and The Joker are cold and heartless, and will stop at no end to achieve their goals, which makes them nearly unstoppable. They are in a completely different category from the rest of Batman's villains. You simply can't get any bigger than the villains in these first two Batman movies--any other villain would simply be a joke. And since any Batman writer would be crazy to even touch the Joker again with a ten foot pole after Health Ledger's incredible and nearly irreproducible performance, Ra's Al Ghul is the only villain who could finish the trilogy off with a bang after the huge scale of the previous two movies.
The first thing I get whenever I bring up Ra's al Ghul as the villain is "No way! He's dead!!" If there's one thing that my movie-viewing experience has taught me, it's that you never believe that someone's really dead until you see a corpse, and even after that be skeptical. The last shot we see Ra's al Ghul in "Batman Begins" is as he slowly shuts his eyes as the Wayne monorail that he's on plunges off the tracks and explodes. Does that make him dead? Not in the least. "Always mind your surroundings", he constantly reminded Bruce in "Batman Begins". Do you think he could've been on that train for such a long period of time and not known the quickest way out if his plan had failed? He's one of the smartest men in the world, not to mention a master of almost every martial art known to man. I'm SURE he had a way out before it exploded. Not to mention that he had already faked his death earlier on in the movie: what's to stop him from doing it again?
And if the writers can't get creative with that, there's always one aspect of the character from the comics that wasn't even mentioned in the first movie: the Lazarus pits. Like the Biblical character they take their name from, the Lazarus pits are secret, mystic underground pits that allow Ra's al Ghul to come back to life if he's placed in them within a certain time after he dies. Because of these pits, he's been able to live for thousands of years (something that WAS mentioned in "Batman Begins"). I know the writers are going for more realism with this particular Batman series, but I don't think that this is much more far out as other concepts from the first two movies, such as the Scarecrow's "fear toxin". Either way, there are multiple angles that the writers could take that could easily (but not cheaply) explain how Ra's al Ghul is still alive.
Another crucial element of this series that makes Ra's al Ghul perfect for a return visit is his incredibly close ties to Batman's origins. This is the only Batman series (including the comics, the animated series, the 90s movies, etc) that credits Ra's al Ghul with Batman's origins. Who is a more worthy villain that someone who was responsible for the creation of the hero, and one that taught him everything he knows? Ra's al Ghul is not only physically able to stand up to Batman, but they also connect on a much deeper mental level. Also, I get the feeling that the writer's aren't trying to make these movies separate "adventures" of Batman, like the old movie series and animated series were. These movies seem to be more of a saga--they're THE story of Batman, not separate stories of his adventures. Because of this, you don't need a bajillion new villains in each movie, but Ra's al Ghul could represent the villain of the entire saga, like Emperor Palpatine is to Star Wars and Sauron is to Lord of the Rings. I really think it will be revealed that Ra's al Ghul was not only behind the events of "Batman Begins", but he also played a pivotal role behind the scenes in "The Dark Knight" and even in the early parts of "Batman 3". Ra's al Ghul is a master plotter and manipulator. What he did in Batman Begins was child's play compared to what he has done in other Batman series. I think that once the trilogy is over, you'll be able to step back and realize the big picture that Ra's al Ghul has been behind all along.
Also, getting Liam Neeson back will most likely be not much of a problem. It's more than likely that he won't appear till 2/3 of the way through the movie, as one final "revelation" that Batman needs to defeat. After the success of The Dark Knight, "Batman 3" is going to be a guaranteed multi-million dollar blockbuster, and I'm sure Liam Neeson knows this, even if he doesn't appear on any of the movie posters or teasers to try to keep it a secret as much as possible.
For these reasons and more, there is really no other choice for a major villain in "Batman 3". Sure we'll probably see one or two more "classic" Batman villains. Now that Rachel's gone, I'm sure we'll probably see Catwoman in the next movie to spark the romantic interest. But Catwoman isn't a master plotter or evil in any sense of the word: she just likes to steal things for fun. There's no way she could hold up a whole movie as the villain. There's simply no other villain that could send the series out with a bang and prove as much as a threat to Batman as either of the villains in the first two movies. Because of this, it's gotta be Ra's al Ghul.
[Update 03/12/10] In a recent interview with Christopher Nolan done in the Los Angeles Times about the new "Superman" project he's starting, he had this to say about the third Batman movie:
“Without getting into specifics, the key thing that makes the third film a great possibility for us is that we want to finish our story,” he said. “And in viewing it as the finishing of a story rather than infinitely blowing up the balloon and expanding the story."
Also:
"Unlike the comics, these things don’t go on forever in film and viewing it as a story with an end is useful. Viewing it as an ending, that sets you very much on the right track about the appropriate conclusion and the essence of what tale we’re telling."
This seems right on target with what I said earlier in this post. The third film WILL be the end of the trilogy, and thus Nolan says that he does not want to "infinitely blow up the balloon" of the story. It's further evidence to suggest that he won't introduce any major new villains, and attempt to tie together what he already has in place. It seems that (like I said earlier) the three films are meant to be viewed as three acts in one larger story. Introducing some other major villain wouldn't serve this purpose, as it would only seek to drive the story forever forward. That's why I really think that Nolan is going to make the best use of the villains he already has, and since the Joker is almost positively out, that leaves us with only one major villain left: Ra's al Ghul.
