This has already been brought up before and several examples were brought up in response, most notably the way Nolan used Joker. He took someone that shouldn't have been convincing at all and made him the main focus of everything. Every single character in "The Dark Knight" was some how effected by the Joker's plots and schemes. He took Gotham City and shook it upside down.
Selena Kyle on the other hand I could see as being a highly skilled cat burglar who likes stealing high profile museum pieces or jewelry along with other material things for her own specific agenda. She could be a socialite by day with a cosmetics company that is looking to make some kind of deal with Wayne Enterprises to boost her profile (thus this is how she meets Bruce) with a checkered past that is linked to the Falcone's and Gotham (i.e. Year One and then Catwoman in Rome). Nolan will find a way to make her work.