Let's see, these are the number of gyms that each type has:
Rock: 3 (Pewter, Rustboro, Oreburgh)
Fighting: 3 (Cianwood, Dewford, Veilstone)
Electric: 3 (Vermillion, Mauville, Sunnyshore)
Water: 3 (Cerulean, Sootopolis, Pastoria)
Grass: 2 (Celadon, Eterna)
Fire: 2 (Cinnabar, Lavaridge)
Ghost: 2 (Eurecteak, Hearthome)
Psychic: 2 (Saffron, Mossdeep)
Ice: 2 (Mahogany, Snowpoint)
Steel: 2 (Olivine, Canaclave)
Flying: 2 (Violet, Fortree)
Normal: 2 (Goldenrod, Petalburg)
Bug: 1 (Azalea)
Dragon: 1 (Blackthorn)
Ground: 1 (Viridian)
Poison: 1 (Fuschia)
Dark: 0
I think that Dark is a definite, GF would have to be brain-dead to leave it out again. It'll probably be a late gym as Dark-types are generally quite strong and a Dark-type leader is likely to use complex tactics, though it could work as an early gym. I think we are very likely to see Bug, Dragon, Ground and Poison as we've only had one of each, and none in the last two generations. I'd like to see a late Bug gym and an early Dragon gym, that would be interesting, people would think Bug=weak and Dragon=strong, but a gym leader with Heracross, Scizor and the like would shut them up. Although we did just have a Bug E4 member, so maybe a middle gym is more likely for Bug.
We're unlikely to see any of the gyms that already have 3 to their name, that would be overkill. So we have the 8 with 2, competing for 3 spaces. It could be any of them, but logic hopes that we won't see any of the same gyms we saw in the last generation, therefore it'll be 3 of Fire, Psychic, Flying and Normal. Now Psychic has been overdone all around, with 2 E4 members to its name, but I suspect we might well see a Flying and/or Normal-type E4 member, as they, along with Grass, are the only types not yet represented in the E4, and so are unlikely to have a gym as well.
Of course, this is all assuming that they keep to the monotyped gym formula. It's likely that they will, but this generation does seem to be different from the others in many ways, and we keep getting the message that these games will be different from any other game before, so anything is possible.
This is also assuming that GF have a shred of logic to their name, which is debatable.
So, my top three wishes:
1) Dark-type gym, 'nuff said.
2) No Fighting-type gym leader OR E4 member please. So far we've had 2 E4 members and 3 and a half gyms. That's quite enough, thank you.
3) Please, god please, no Rock gym. And if there is what, can it not be the first gym? We've done it three times already, any more would be simply ridiculous. It's boring, repetitive and uninspired, and we've had enough of it.