Well seeing they shown that the Dragon guy is *somewhere* in the story as well (and he's just a borderline character in S/M), and the Fairy chick also gets a trial of her own, it is possible Kahili also has a appearance somewhere.
What if she actually gets to do a flying trial for example?
We've seen a quick glimpse of Kahili, so she is in the game. It was at the
0:13 mark of this video. Looks like she's on a dirt path/in the cave and is showing the pose for the Flying-type Z-Move.
Though as a Trial Captain? Well certainly a possibility, but at the same time that would mean they'd have to replace either Lana, Kiawe, or Mallow (or be adding a fourth trial to Akala and eighth overall). Also Captains can only be Captains till their 20, and I think Kahili is around that age.
If they want to significantly include her in as someone other than an early cameo of an Elite Four member I can think of two ways: they're going to do the Pokemon League thing and her gold course is her "Gym" (though that doesn't explain why she's on a dirt path/in a cave doing her Z-Move pose, you'd think that would happen on the green turf) or maybe we're getting a new set of Island Kahuna. I'd rather the League idea, they can even have some Captains become Gym Leaders so their trial is the Gym Puzzle before we then later battle them directly. Or maybe they are adding more Trials, nothing said their only needed to be seven (and they may raise/get rid of the age limit thing, who knows, maybe they'll make the Trials into the replacement for the Gyms just for Alola).
A golf course does seem like the perfect spot for a trail...
Kahili: Okay, for your Flying-type Trial, you'll need to be a caddie for the day at my dad's resort. What? Being a caddie teaches plenty of life lessons. Patience. Diligence. Thriftiness. Now get out their and wing it.
(Spend most of the Trial finding and fetching golfer's balls, usually being attacked by a wild Pokemon. You meet Kahili on the final hole)
Kahili: Oh good, just in time. I got tired of waiting so began practicing my swings. (Does a swing and sends the ball flying). Look at that birdie go. Though I may have overshot it. Caddie, go fetch it, would you? I promise it won't be a waste of your time.
(Fetches ball near a cave and, surprise, attacked by a Totem Oricorio that summons the other style of Oricorio)
Kahili: Good job! See, I told you it wouldn't be a waste of your time. Congratulations. You passed the Flying-type Trial. Here's Flyinium Z, to use it pose as I do. (Does pose). You know, I'm quite the accomplished trainer myself. Ascend through the ranks and maybe we'll one day have a battle.
Since we’re getting other/different trials, perhaps we’ll get a different Elite 4 challenge at the end of the game as well, like we’ve traditionally gotten in past games.
Actually, if they change the Elite Four, I think that would be the first time they ever did that. We've only ever gotten different Gym Leaders or Champions, the Elite Four is usually left untouched (and underutilized. It's so odd that previously to Sun & Moon they often neglect to do much with the Elite Four despite being the four most powerful trainers under the Champion of the region. Heck, the only reason the Sun & Moon Elite Four stand out is because they were Kahuna/Captain we met before except Kahili, and they did very little with her so the trend follows).
