Well, it all depends on whether Serena will get a cameo on SM or not. We shouldn't disregard the possibility so quickly because Cilan didn't appear until the very last episode of XY&Z.
I don't have high hopes, but we can't say for sure it won't happen.
Now, if she does get a cameo, then i guess either Iris, Clemont or Bonnie. Any of them is fair game at this point. Iris only got a special episode, but so did Clemont and Bonnie. I believe Iris would be the top priority though, since it has been a long time since BW, longer than XY at least.
Regarding the SM companions, it will depend on whether they will get their own special or not, but regardless of that, they can wait a bit, just like the others are still waiting for their appearences.