Who's Batman?

From the very beginning, Watchmen tries to be the natural extension of superhero comics in the real world.  That is, Watchmen discusses what the world of superheroes might become 30 years down the road from when Superman was fighting Lex Luthor.  It takes on a very similar quality to that of The Dark Knight Returns.  I think that because Batman seems to be the most probable or realistic superhero, I think that he is the best character to include in these types of comics.  But who is Batman?
Several characters in Watchmen could be the probable extension of Batman into the real world.  First (and probably most obviously) is Rorschach.  He is a ruthless crime fighter who seems at ease going outside the law to get what he wants. Given enough time, it is possible that Batman could devolve into a hero like Rorschach.  Another could be Nite Owl or even the Comedian.  However, it is not as important to the story that a single character can be seen as the natural extension of one of our famous superheroes. It's much more important that we can make these connections at all.
Watchmen is  written in such a way that it is ease to see it as the intersection between our world and the worlds in superhero comics. What is so well done in Watchmen is that the reader can identify the characters they know from their childhood in Watchmen.  It is almost like the superhero genre has grown up along side its reader.  Being able to identify known characters in Watchmen makes this idea much more clear.  By seeing the extensions of characters we are very familiar with, we get a sense of realism in the graphic novel.  This was much more explicitly done in The Dark Knight Returns, where we see the literal extension of Batman into a world after superheroes.