I think (since I did a water gym) I'd go with a Fairy trial (I know there is a Fairy trial in the new games, it was a great trial and a PAIN IN THE TAIL). My "trainer" will be a girl because that is what I mostly play as these days (2017 baby) and it would be right after the last trail of Island three, right before the Grand Trial. All of this I thought off the top of my head.
I would have the trial be in the Malie City Gardens on the third Island. Basically, you know how there is a Jhoto theme from the city and the gardens? My trail would be in the big tower in the North East. My trainer would meet you in an average everyday attire in the front and would talk about how she moved to the region from Jhoto at a young age and her mother was a Kimono dancer in Ecruteak. She would start to talk about how she became a trail captain but would apologize for going on too long. She would ask if you're here for the trial, and if you are, she'd say that she "isn't prepared" because she likes to get dressed up for Trials because they're special occasions. Her friends borrowed parts of the outfit she likes to wear, so she asks you to get it for her.
The trail would consist of you finding three girls who are of the trainer class Furisode Girl from X/Y in the Garden. When you find them, you have to beat them in a battle, with them having 2-3 Pokemon each. If you leave the Garden, which the trail guide will be there to check, you forfeit and have to start over. Each girl has part of her outfit, the dress, the shoes, and a headband. When you have them all, return to the tower. There, we'd go inside and my trainer would go to a room to change where she can still talk to you. She'll ask if her friends "bothered you" and that "they all just love to battle." She would be changed into her kimono girl outfit and start to dance. During all this, a misty terrain would begin to appear and if when your trainer asks, she says it's just to "relax" and eventually, she summons the totem Pokemon for my trial, Sylveon!
The Sylveon would have an omni boost and be level 40. Sylveon would have cute charm and be female for all those male physical attackers, and misty terrain will be on the battlefield during the battle so you can't use status conditions. Sylveon's moves would be Moonblast, Draining Kiss, Shadow Ball, and Psyshock with the item sitrus berry.
It's ally Pokemon would be a level 37 Comfey with Triage, knowing Draining Kiss, Floral Healing to heal Sylveon, Grass Knot, and Synthesis. Basically healing itself and Sylveon constantly with Priority. And its second ally would be Mawile level 37 also, with intimidate, knowing Play Rough, Iron Head, Sucker Punch, and Fire Fang.
If somehow you get through that trial of rage, you'll get Fairinium Z, the mist would disappear and my trainer would praise you for your battling expertise.