Like Noxus, I think we're getting 4 updates when Kai'Sa comes out. 5, if counting Kai'Sa. Judging by what her story may involve, I think the Void's theme will be corruption. Malzahar embraces it, while Kassadin resists it. I think Jax, and maybe Zilean will gain an update, but we could see void creatures instead.
I think Kassadin is a lock because of his relationship with the new champ.
Malzahar stands a very good chance as well, not only does he have prior history with Kai'sa, he's one of the most prominent Void characters.
For the other two, I think it depends on the direction they'd want to take. If it's champs with Void interactions, but not Voidborn, I think you're right, Jax and Zilean. If they want to focus on the Void itself, I'd like to see some love for Cho'Gath and Kog'Maw, because they feel very underdeveloped right now.
I would personally prefer to see updates on the human folks who have been touched by the Void in some capacity. Jax, Malz, Kass, and Zil. A short story on the last night of Icathia would be pretty rockin'.
I feel like Kog and Cho's long lore are best left alone until they get their VGUs, since they're both on the priority list.
& 
will certainly get an update considering their link to Kai'sa.
If they wanna focus on Icathia,they could update 
& 
for the last 2.
If they wanna focus on the voidspawn for the other 2 then I think 
has a good chance,being the first void creature on runeterra and either 
(assuming he is still connected to malzahar) or 
(if they want to avoid kog'maw because he is on the vgu-list).
