Actually if we're travelling back to Kalos in S/M we can discount the Kalos gyms types, as I don't think they'll double dip. They certainly didn't with GS.
That's a very interesting point. If Sun & Moon visit Kalos again and there are no changes to the Gym Leaders (or rather, no changes to their Types), then that leaves Normal, Fire, Flying, Poison, Ground, Water, Dragon, Ghost, Dark, and Steel. If we exclude the Elite Four Types, that would leave us with only Normal, Flying, Poison, Ground, Ghost, and Dark. So there'd have to be some overlap, at least with the Elite Four. I actually wouldn't mind if Valerie moved up into the ranks of the Elite Four to give us our first Fairy-Type Elite Four, and I'd love to see a Normal-Type Elite Four, too.
So I guess my own personal preference would be something like this:
1. Dragon(I feel this would be interesting since they would resist your starter, making for more of a challenge)
2. Flying
3. Ghost
4. Poison
5. Ground
6. Fire
7. Steel (or no Type)
8. Dark
(I wouldn't mind seeing them attempt a no-Type Gym again, but if they do, I'd hope it's 7th so there's more time in-between that Gym and the Champ, who will likely also be no-Type)
E1. Normal
E2. Water (Siebold was my favorite Kalos E4 so I'm biased)
E3. Fairy (could possibly be Valerie)
E4. Electric (could possibly be Clemont)
No-Type Champ (may or may not still be Diantha)
K1. Bug
K2. Rock
K3. Fighting
K4. Grass
K5. Electric (if Electric E4 is Clemont, this could be Bonnie)
K6. Normal (replaces Valerie)
K7. Psychic
K8. Ice