Let's imagine a world where Pokémon do not exist, but every human has a latent power, associated to one of the 18 known pokémon types.
With trainning, each person could awaken these powers and benefit from the abilities associated to their type and use their type's attacks.
For example, if your latent power is Psychic, you could benefit from an ability like Oblivious, or use moves like Psycho Cut or Teleport.
There is no type effectiveness, so everyone fights on equal footing (However, certain attacks will still come on top against another, ex: Hydro Pump vs Flamthrower).
The question is simple: What type would you want to master?
To help you, here are some descriptions of how each type could play out:
-Bug: A more technical and ranged type than physical. Sure, it doesn't lack physical options (X-Scisor and Steamroller come in mind), but the various buffs and debuffs offered by Bug-type moves encourage ranged attacks, defense and speed more (Quiver Dance, String Shot, Sticky Web, Defense order, etc.)
-Dark: A type that forces the fighter to fight in a dirty fashion. With moves like Sucker Punch, Thief, or Feint Attack, the opponent is in for a frustrating match. However, while it kind of lacks ranges options (Dark Pulse and... Snarl?), it gives Nasty Plot.
-Dragon: The most straighforward type and the most aggressive. With the only buffing move being Dragon Dance, the Dragon fighter is pretty much forced to play it fierce. Luckily, it has quite a lot of good offensive options, like Dragon Claw, Dragon Pulse or Zygarde's move (forgot its name)
-Electric: With few buffing options, this type compensates with various other strategic possibilities, like Electric Terrain, Thunder Wave, or Magnet Rise. Its quick and powerfull attacks will certainly proove troublesome.
-Fairy: The type with the biggest lack of options. Few buffing moves, but some very powerfull attacks (including one which heals the user) and Attract can be quite effective, and Moonlight is a usefull recovery attack.
-Fighting: A type to use with discipline, requiring finesse and strenght at the same time. With Bulk Up to boosts their physical attribute and attacks with various side-effects (Power-Up Punch, Aura Sphere, Brick Break, to name a few), a Fighting-type fighter is pretty tricky to fight. Detect is a fine addition.
-Fire: Very few if not no buffing move whatsoever. However, it has the capacity to prove strategic-minded in battle with Will-O-Wisp or Fire Spin. Of course, one of the best ways to fight as a Fire fighter is to play aggressively.
-Flying: A pretty much ranged-oriented type in general, but doesn't lack physical options. Moves like Mirror Move and Roost are prety usefull, but then again, not much buffing options.
-Ghost: A pretty mind game-oriente type, disappearing in the shadows with Shadow Sneak, putting a curse with... well, Curse, or confuse opponents with Confuse Ray.
-Grass: Pretty much health-oriented, with various options to stall the ennemy (Leech Seed, Giga Drain, Synthesis) and has some pretty strong moves to keep their opponent in check, like Leaf Tornado or Leaf Storm.
-Ground: Ground-type fighters do better when using the very fight arena, digging through the ground, shaking the floor, or generally messing with their opponent's footing.
-Ice: The Frozen status is pretty cool, but otherwise this type allows to create ice on the battlefield, mess with the ennemy's vision with a snowstorm and striking with Blizzard, without forgetting Aurora Veil.
-Normal: The sheer absurd number of possible attacks makes this type the most dangerous one to face, with moves like Protect, Recover, etc.
-Poison: The dirtiest type, leads the fighter to poison their opponent and to stall them or worse use the poison to hit even harder!
-Psychic: Personnal favourite. The psychic type, in addition of moves which allow to boost all stats (Cosmic Power, Calm Mind, Meditate, Agility), grants the ability to teleport and allows the use of Light screen and Reflect. Moreover, it has great ranged options, though its physial options are rather slim.
-Rock: Surprisignly, this type would be more like a glass canon that a mighty glacier. Effectively, its only buffing move is Rock Polish. The ability to create sandstorms and to throw rock at your opponents is pretty neat.
-Steel: An all-around excellent type. While lacking Normal's unpredictability, it has King's Shield for defense, Autotomize and Shift Gear for buff, and plenty of good choice for offense.
-Water: While the ability to create rain is good, the Water type, just like Fire, lack any sort of usefull buff (except Withdrawl). However, Aqua Ring is a nice defensive plus, and it has plenty of side effects to its offensive moves to adapt its strategy in battle.