Fan fiction is rarely all that interesting to anyone other than the author... that being said...
I think it would have been great for us to meet Selina as she is being let out of prison. Maybe as she sits before a Judge who is explaining to her the rules of her release. We see Holly / Jen as they reunite.. Selina has been locked up for a number of years - a victim of the Dent Act's harshness. She was caught stealing - big time stealing. But not just frivolously. She was taking down a big organization - like a Cobblepot / Deckard type systematic white collar criminal organization... one that had wronged her personally somehow. She was taking down the bad guys in her own way... but got caught.. and though Gotham appears cleaner... safer... it is still being run by the criminal and corrupt. She was locked away and they criminals kept operating in broad daylight.
Now she is out. The details of her release STRICT. She can't get into trouble. But her being Selina, she wants to pick up right where she left off. Taking down Gotham's phony upstanding citizens. She sets her eyes on a particular prize - Holly warning her that she can't. If she gets caught, or even if anyone see's her... it's over. She's done for. Selina can respond with something to the effect of: Don't worry, nobody will see Selina Kyle anywhere near the blank blank blank...
Of course, this gives her cause to create the identity of Catwoman (in fact, the above conversation between her and Holly can be going on as the TV broadcasts a debate featuring Mike Engel, The Mayor, Gordon, etc etc as they discuss the BATMAN... her creation of this CAT WOMAN can be directly inspired by this BAT MAN). And thus, CATWOMAN bursts onto the scene in a big way.
She could align herself with BANE, who she would see as somebody willing and able to expose the criminal and corrupt of Gotham just like she wants to. He can reveal to her the fact that Harvey Dent was no hero.. just another Gotham power player that turned on his city.. and they hold him up like an idol. Using the lie of his downfall as a means to stretch their power further. Locking away those who get in their way. This pits CATWOMAN against BATMAN directly - as she herself was locked away and kept there based on the Dent Act. Her desire to expose this lie and to destroy BATMAN for the role he played in her incarceration make her align fully with BANE.
Meanwhile, Selina Kyle and Bruce Wayne come into contact somehow (perhaps she is doing recon - or perhaps bringing the financial stability of Gotham crashing down is still in play, so she is tasked with getting something from Wayne (fingerprints, information, whatever). She see's him as just another of Gotham's elite - wealthy beyond reproach and indifferent to the real suffering present in Gotham (they're masquerade ball can still be in here word for word... it's a fantastic scene). But Bruce opens up to Selina.. he shows her something - a plan for Gotham.. a better world for Gotham. Perhaps he invokes the "I Still Believe in Harvey Dent" motif, which she scoffs at and tells him not to believe what he reads in papers coming out of Gotham. He tells her that who Harvey was, what he represented to a better Gotham, a safer, fairer Gotham is too powerful and too necessary to be brought down by skepticism. Or even, facts. It doesn't matter who Harvey was... what mattered was who he was to this city. Who he can still be. He succeeds in convincing Selina that he is one of the good guys, though she doubts his good intentions are enough to save Gotham. Luckily, there are those with more than intentions doing something about it...
When CATWOMAN and BATMAN meet - she is fired up. They fight, banter, argue. CATWOMAN'S sexiness, her flirting is more prevalent. She toys with BATMAN. (A perfect mirror to the Bruce / Selina masquerade dance). They fight and she leaves him, cocky and confident as she slinks away into the night. Later, when CATWOMAN is doing some recon for BANE - stealing something - she gets herself caught in a sticky predicament and who but BATMAN comes to her rescue. She isn't thankful. But they connect on a different level. Talk. It is enough for her to come to him on their next meeting saying "You helped me, I thought I'd return the favor. I hear you're looking for.... (something). She tells BATMAN she knows where it is and of course, leads him right into BANE'S trap. She rubs it in harder here than in TDKR... telling BATMAN why she betrayed him... what he and his lies did to her. How blah blah blah..." but of course, when Wayne and BATMAN are revealed to be one, she has her whole view thrown out of whack. She suddenly realizes she was wrong... but she's in too deep.
After BATMAN is broken, she lingers around, too deep to just run. She is revealed that BANE has no interest in plucking the privileged from their perks and brining justice and fairness back to GOTHAM... he want's to destroy it. In an impulsive CATWOMAN way, she draws a line in the sand, escaping BANE and his men and is suddenly stuck to help clean up the mess she made. She finds GORDON and comes to his aid - suddenly forced to work with those that put her away - those she considered the true criminals of Gotham. She acts as a BATMAN proxy while he is in BANE'S prison (an awesome mirror of the BATMAN - HARVEY - GORDON rooftop scene from The Dark Knight can occur between CATWOMAN - BLAKE - GORDON, where CATWOMAN is now BATMAN, minus his code and with a much darker edge and grayer center. Where Blake is HARVEY, the hope for Gotham's future (good cop in a sea of corruption - also forshadowing his taking over the mantle as Gotham's protector) and GORDON is GORDON, our lynchpin.
CATWOMAN aides GORDON and GOTHAM as she can until BATMAN returns and they fight side by side. I still think she should indulge her selfishness and impulses more to her demise so that it is not a happy ending for Batman and Catwoman (Bruce and Selina) but that she is left to answer for crimes - regardless of how she views them. She is BATMAN without the code - her downfall because of that validates his having one. (Her crime can be her finishing bringing down the Cobblepot / Deckard organization that sent her to prison initially).
And there should be no BOMB. Tired and lame. BANE could talk about a bomb.. but the bomb could be the reveal of Harvey Dent.. living, and obviously as Two-Face. Exposing him and the truth behind the lies GOTHAM has been fed, plunging the city into riots and chaos. Forcing GOTHAM to tear itself apart - much like the League tried to have them do in Batman Begins, but this time, using corruption and seeming logic to feed their distrust and anger instead of chemicals.
I love Catwoman and Selina and Anne in TDKR; love her "oops" intro, love the bar shootout (vacillating between in charge and faux-terror), love the masquerade scene, and love her Catwoman... but that's the problem with TDKR on the whole... lots of awesome elements that don't add up to an awesome whole. In my opinion anyway.