Yeah, Kristian will come back when/if the show ends as a gesture to Corday and her friends at the show. She doesn't need it.
Freddie Smith will begin lobbying to come back/crowbarring the studio doors when he realizes he is not going to become a lifestyle brand.
They didn't know what to do with her in her last few returns but she did fine acting-wise.
Billie was always very much written - at least when she was playing her - with LR's spunk so I am not worried about her acting.
I am pretty sure she will be back (I mean, Reckell left in a blaze of bitter glory and STILL came back) but I don't put it on the same level as FS.
KA won't ever *need* to come back, financially or otherwise. On the other hand, whenever she does want to come back, she will be welcome.
FS is very very likely to need to have a job at some point and prefer *that* entertainment job over having a real job once he realizes that pyramid schemes and YT money wear off the longer he has been off the show and his "fame" fades. But I can't imagine that he will be particularly remembered/missed in 10 years if it takes him that long to see the light.
The iron is still warm but it will get a lot colder a lot faster than KA.