I can see multiple changes with Gym Leaders either leaving altogether, getting promoted to Elite Four, or maybe even relocating.
Cilan, Chili, and Cress - Too me they seem like the easiest to replace. In the anime both Cilan and Chili aren't even running the game as they both went on their separate travels. And obviously with Shizui we don't need another Water leader, so I don't see a use for Cress either.
Lenora - I can see her attended to work at the Museum, so I don't necessarily see her too be a gym leader. I would prefer a Grass type in Nacrene since Pin Wheel Forrest is around there and other forest area as well.
Burgh - I'd like to see him relocate like Blaine did. Obviously the circumstances are different but he seemed out of place in Castelia. I'd rather see him in another City or Town. Anville town may be good for him since he is out there with a lot of wooded area.
Elesa - I wouldn't mind her staying out Nimbasa gym, just rework her team for heavens sake. Two annoying Emolga seriously. Give here something like Ampharos, Eeleltrik(since she had Tynamo in the anime) additions to her team and cut one of the Emolga.
Clay - He can retire as a wealthy tycoon and maybe loan his place out to a Rock gym which is what I would prefer to see there. I don't see him or care to see him play much of a role as a leader personally.
Skyla - There is another airport in East Unova on the dubbed "Grey Mountain". So they could relocate her there if they wish. I still see her staying in Mistralton.
Brycen - I honestly don't know what he will do or what his role will be. But the key pokemon in this game is Kyurem which is an Ice poke, so I can see an Ice type either Gym Leader or Elite Four member, so him staying I can see that happen.
Drayden - I always felt he would be promoted to Elite Four, but then again he could also be stuck being the mayor of Opelucid City and still be leader there.
Iris - Also being another anime main character, I don't know what she could be doing then, but I can see her be in Opelucid as well.
I can see 16 gym leaders, but that may be a stretch until we get farther info.
First two seem to be Homika as first gym leader and Shizui as the second one. So lets hope we get farther news soon. And hoping we get gyms in places like Anville, Lacunosa, Undella.