When I think "lead" I think of someone who makes it to the cover of the DVD or is featured on a poster. It's interesting that in Begins, neither Ra's, Scarecrow, or Rachel were able to make it to any image meant to represent the film... but in Dark Knight, there are a couple of different Joker centric or featuring posters, covers, promo sites, etc. So, in that sense, I think Batman needs supporting characters, definitely, and those can be female or not... but I don't know if he needs a female "lead" (using my narrow "poster-worthy" definition) of Joker prominence necessarily.
I'd want to tell a story that's a thematic progression from Begins and Knight but it's own and different story, and then find characters that fit rather than trying to jury rig a story around a character wish list. Begins is about the psychology of symbols and Knight is about how symbols escalate conflict and require hard choices (be it a one boat or another, one tied up victim or another, big brother cell phone surveillance, or being portrayed as the villain) where "right" is ambiguous or conflicting based on the measure (utilitarian v. moral; one life v. other; useful lie v. hurtful truth; etc) ... so the next step is what? I don't really know. If I had to guess it would be about humanity. Bruce exchanges it for the symbol in Begins and he makes soul wrenching compromises for it in Knight... so the third might be trying to not let the symbol consume and completely warp the necessary human core?
If that's the case, a female super villain love interest could match that theme because it hits both barrels... the human part that still needs and wants love, but the symbol part which is the only reason the Batman is coming in contact with this crazy person and the large part of his appeal to her. The weakness of this, of course, is that it's a little too love-centric for a character that stuffed all that down with Rachel and felt real loss there... and, as the story is told, the only reason she gets in and all the other flings don't is because she's a long-time childhood friend that knows him from before. I suppose you could play it as Bruce becoming, entirely, the Bat therefore it is Batman falling in love with Catwoman / Talia / etc. and not Bruce (and that, ultimately, as finale to the trilogy, Bruce has to "beat" Batman and reign him in for the purposes for which he was intended - to not let the symbol grow malignantly out of control- as with the copycats or mirror in Joker- in his own life)... but that feels like it's been done before?
The Nolan Batman is such a discrete and rational world I have trouble imagining the organic entry of a new love... and it's story first for me, so I'm skeptical, but I can see a path to it.
If trying to hit a character checklist, Catwoman is obvious but pretty rote by now. Vicki Vale is a traditional comic character but comic history doesn't really matter (see: Rachel / Ra's / etc). I really don't know any character that's a "must see".
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