I have no doubt that Iris will come back for at least one episode. I just hopes she comes back with a new Pokemon or two, or even evolutions.
It is a tradition as far as I'm concerned, though I suppose not an official one.
Cilan coming back isn't as certain. We never saw Brock throughout the BW run, just in a post BW Japanese special that never hit America. Aside from a few background things we never saw Tracey. And Max never made a comeback at all, even during May's return.
I have to say in some ways I'd like to see all the girls make a return for a bit, if for no other reason than to show off some mega evolution. Iris aside each of the preceeding girls have at least one Pokemon capable of mega evolution, though Dawn's Buneary would have to evolve to Lopunny first.. As numerous as these things are getting (not to mention Ash already has three Pokemon capable now of this) I wouldn't be surprised to see mega evolutions being shown more in the anime even for main characters.