According to exec producer Gary Glasberg, the plan is to feature a handful of guest agents in the same way Paul Cassidy (Jessica Steen) filled for Kate until Ziva gained acceptance or the way rotating interns replaced Bones's Zack Addy (Eric Millegan). I assume from the amount of detail given about Agent Bishop, she will eventually be the new full-time team member:
[Bishop] is bright, educated, athletic, attractive, fresh-faced, focused and somewhat socially awkward. She has a mysterious mixture of analytic brilliance, fierce determination and idealism. She’s traveled extensively, but only feels comfortable at home.
The description sounds a bit scattered, but clearly geared to appeal to younger female viewers. NCIS is still tops in the ratings going into its eleventh season, but even the best need occasional shakeups.