Bratty Kid: Look similar in style to Youngsters, but have a devilish smile as they send out their annoyer Pokémon; Pokémon like Shedinja, Chinchou, and other 'mons with well-known annoyer movesets or just generally annoying Pokémon all come into play.
Landscaper: Older males often driving lawnmowers or weedwhackers in forests or in city parks; focus on Bug and Grass types that use Cut and Slash as well as Bonsly, although in some variations they might have Water and Rock types depending on what sort of landscaping they're doing.
Soldier: Playing off of Lt. Surge, soldiers would be similar to Sailors in their brute force Pokémon like Machop, but differ in using electric types as opposed to the water types of Surfers. It wouldn't be unusual to see them using Nidorino or other ill-tempered Ground types, either. You'd probably either see them in a Gym, or they'd approach you and claim you'd ruined their camoflauge, or messed up his mission by getting in his way.
Big on paralyzing foes.
Lawyer: As a sort of a younger counterpart to Gentlemen, you'd see both male and female Lawyers using Growlithes and Pokémon indigenous to the area, but never particularly fierce, as they'd have to behave themselves in the big buildings of a city.
They'd use a lot of unusual moves, like Lock-On, Odor Sleuth, and Mean Look, as well as Pokémon with unique abilities, so it'd feel like any battle with them would have as many clauses as any legally binding document. Big cash rewards, even though it's a simple battle.
Gym Fan: Gym Fans would be similar in nature to Pokémon fans, but be more a fan of the trainer than the Pokémon themselves. You might see a Gym Fan corrolating to a specific Gym several cities after you yourself beat the leader, and they'd unleash Pokémon corresponding to the Gym's type as well as Pokémon of a similar temperament to the leader (if applicable), often getting worked up after they see the badge that proves you defeated their hero.
More often than not, they'll share at least one Pokémon in common with the Gym Leader they adore, and you'll be able to Thief the same TM you got from the Gym Leader from that Pokémon.
Scuba Diver: These trainers show up when you're Diving in deep water, and Pokémon like Tentacool, Mantine, and Wailmer all use Dive on you as well. Rarely seen, and perhaps that's a good thing, since their overuse of Dive can make it a tiring battle.
Treasure Seeker: Either on small islands, or also Diving, Treasure Seekers are amphibious Ruin Maniacs, looking either for ancient caves with unique Pokémon inside, or sunken ships holding treasure. Poliwhirl, Corsola, and Marshtomp are some of their more common obsessions.
Arctic Ranger: In anticipation of a more chilly theme of the game, Arctic Rangers would be Pokémon Rangers almost as suited up as the Ice Climbers (and with identical colours-- as an homage, perhaps?) using Sneasel, Glalie, and the Ice Pokémon of the 4th gen.
Opportunity to Thief Nevermeltices.
Slope Hiker: Appearing in the same areas as Arctic Rangers, they'd have Ice types as well as bulky Fighting types and high HP Normal types that can endure the cold, using a lot of Fighting Moves to break through the environment as well as any Ice types you might have.
Obviously, they're really just hikers suited for the snow. If there's any Rock/Ice Pokémon in the game, they'll be the ones to use them upon you.
Engineer: Steel and Ground types that stand on two legs, and Electric types like Magnemite and Voltorb. Some might have Beldum, but only later in the game.
Big monetary payoff.
Pilot: Appearing in airports along with Flight Attendants, Pilots would, you guessed it, use Flying types. Actually, to clarify here, Male Flight Attendants would use Pokémon like Fearow and Dodrio, with Zangooses and Loudreds as an occasional trump card, Female Flight Attendants would use Pidgeottos and Altaria, with either Skiploom or Delcatty as extras.
Pilots would be rather unconventional in actually using flying types that aren't excessively common, like Skarmory, Crobat, and Swellow, almost as if they were a Gym Leader in themselves.
All would have large payoffs.
Aviator: More 'grungy' wannabe pilots you'd see outside airports, these scarved trainers looking a bit like Factory Head Noland except with brown coats tightly worn, and let loose Pokémon like Noctowl, Dodrio, and Pelipper.
I'm out for now?!?!
They'd use all the "sad/neglected"-themed Pokémon like Cubone, Banette, etc.