The Gayle King Show Isabel said that she was never officially told that she was done with the show and on her twitter page, she has been actively open to the return. But, as it comes down to it, the final decision for Isabel to come back is up to Executive Producer Warren Leight. And believe me, I have spent a few years convincing him to let her come back- at least for one episode. That's all I want!
Here's some of my ideas of how Kathy can return without Elliot:
-In Season 13's Personal Fouls, Olivia packed up Elliot's desk (on Cragen's orders); it's unknown if Elliot ever came by to pick up his belongings although I'm more leaning towards no. Kathy could pick it up and make small talk with Olivia, congratulating her on becoming a Sgt.
-In Season 9's Paternity, Olivia was there for Kathy after their car accident while she was in labor and gave birth to Eli. Kathy kinda owes Olivia a favor...Kathy could be brought in to help counsel in a case as a mother (let's face facts: both Olivia and Amanda are not mothers; both are single and unmarried women); Kathy was even a teen mother! Olivia brought in Kathleen in season 10's Crush to get through a teenage girl who was getting abused by her boyfriend and wouldn't fess up.
-Kathy knows someone who was raped and needed help. Knowing that Elliot wants nothing to do with SVU anymore, Kathy turned to Olivia for help to aide her friend. Kathy doesn't have to have a huge role in the episode, more as a lead-in to help connect SVU to the victim and as a supporter at the trial. Two to three scenes.