They can't even entice Bale to do a Justice League movie so what more for a movie where all he'll do is sit down and ~talk~ for a total of like 15 minutes? Maybe they can do it where Selina is talking to someone that's presumably Bruce but we can only see his back so they can just get a double.
All joking aside, as much as I love the character (she's by far my favorite new character in TDKR), I still vote no for a solo spin-off. For me, she's at her best when she's interacting with Bruce/Batman. It's their dynamic that I absolutely love. But that's just me.