In a perfect world, my idea for Black Cat was essentially this..
-After Gwen's death, peter is more secluded than ever, he sort of gives up being spider-man. Until he runs into a Cat Burglar, (maybe she stole something personal from him or someone he knows (like harry)
(This mourning and black cat combo i'd also ideally include the black suit with, which sort of corrupts peter like a drug, and latches on to him in his most depressing moment of his life. this story arc would also parallel Harry's drug story... but spidey's suit really wouldn't show it's ugly side till about the end (the symbiote saga i'd make a trilogy out of, but 2 films would work if needed as well) )
-Felicia essentially get's him into "peter parker no more" mode, where he's basically Spider-Man full time, the cusp is when he learns how irresponsible to the Peter Parker life he's been, flaking on MJ and Harry and May and what not... his school work, etc..
-Peter figures with Felicia, she can defend herself, and is safe for him to be around. But he's still extremely skeptical of keeping her safe, and refuses to show him his identity. This later causes a rift between the two of them, and Felicia decides to tackle the films villain on her own, but is nearly killed in the process.
-In the end, Peter learns that no matter Peter or Spider-man, Or hero or civilian... people can always get hurt and it's not necessarily always because of him. He also learns to balance the life of Peter and Spidey more properly.
-the twist at the end of course is that Felicia calls things off with peter, leaves the city, and hints that there's probably another girl he knows that he should be dating. and that she's not the settling down type.
-Black Cat is sorta a quasi rebound.. they never really have much romance, she just flirts quite alot and there's a sense of comfort Peter finds in her... But it's not a full fledged relationship (or hook up) like they've been in the books