Weaklings choose the path of the gym leader. Cool cats choose the path of the champion. But anyway, I'd be the Dark type leader, since no one else is doing that. Probably late in the game, since if we've been waiting so long for it it better well be awesome. A team of Honchkrow/Weavile/Tyranitar with two spaces for new Pokémon, and a Greninja gets added for rematches (and Tyranitar will be able to Mega Evolve).
If I were to be a gym leader, I would run a Poison type gym, the seventh gym in the region.
When the trainer first arrives at the city, he sees a grunt from the region’s evil team running out, chased by a Salandit and an Ekans. Later on, an NPC tells the trainer the grunts often try to raid the city but the gym leader and her loyal battalion of Poison types keep them at bay.
When the trainer first arrives at the gym, the building doesn’t look like anything special, more of a small light brown hut than anything. The only thing really extraordinary about it are the Salandit and Ekans sitting in front of the door, the same ones from earlier. When the trainer approaches, you will have the choice to battle them, give them a Berry, or run. If you win or give them a Berry, they will allow you to enter.
Once inside, the gym will take on an ancient temple theme (much like those in archeological movies) with a warm brown color palette, several pillars with Ekans, Arbok, Seviper, Salandit, and Salazzle carved into them, rooms that get bigger and darker the farther the trainer gets into the gym, and of course traps. Near the doorway, the trainer finds a lady that tells them that several of the tiles in front of you hide nasty surprises but if they follow the pattern, they’ll get across safely.
The pillars will shoot flames that light up shapes on the tile, usually of various Poison types. If the trainer steps on a tile that’s not lit up, a battle will be initiated with either an Ekans or a Salandit. Once the trainer gets past the tiles, they will descend a flight of stairs to the next floor.
The next floor will be dimmer and bigger than the first and will be pretty much the same, just more tiles, a different pattern, and alongside the Ekans and Salandit, Spinarak, Ariados, and Seviper can spawn in the tiles not lit up.
The third floor is even dimmer than the last and has a moat around and under the tiles. Once again, the pillars will shoot flames to light up the tiles. Much like last time, if the trainer steps on a wrong tile, an Ekans, Salandit, a Seviper, a Zubat, or a Golbat will appear but this time there are several puddles of water that the trainer will occasionally have to walk through which will trigger a Skrelp or a Mareanie to appear.
The fourth floor will be the darkest and hardest to navigate. When the trainer first arrives, it is completely dark until they step on something which triggers a battle with a Golem. Once the Golem is defeated, the tiles will be lit up into the hardest pattern yet. If the player steps on a wrong tile, either an Ekans, a Salandit, a Spinarak, an Ariados, a Seviper, a Zubat, a Golbat, or a Graveller will appear. On the last line of tiles, none of them will have been lit up and the trainer will have to walk on them to proceed. This will initiate a battle with a Nidoqueen. Once the trainer defeats the Nidoqueen, a doorway will open in the wall and they will proceed through it down a hall lined with torches and Salandit until they reach the gym leader.
My gym team would be: Salazzle: My ace Pokémon. Moveset: Captivate, Venoshock, Flame Burst, Dragon Rage Arbok: Moveset: Wrap, Crunch, Venoshock, Toxic. Nidoking: The first Pokémon to be used. Moveset: Horn Drill, Megahorn, Toxic Spikes, Earth Power. Mareanie: Moveset: Bite, Spike Cannon, Recover, Poison Jab.
I've got a few different mono teams (Water, Ghost, Ground, Dragon and Dark) but typically I gravitate to Ghost & Water. And just to be different, I choose Ghost If I was a Ghost Gym Leader, I would use the Ghost Team I used in Gen IV (see below).
Both the Gen IV regions have Ghost Gyms already, so whilst I love Sinnoh, me and my fiendish friends would spread our wings and set sail to the Kanto region. I heard the Viridian Gym Leader left to run a battle facility for advanced trainers in a far off region, so there's I'm sure there's room.
I'd love a gym in Lavender town, but it doesn't feel very respectful to build one there. So instead I built one on Three Island, in the gorgeous Sevii Isles. Sevii isles 1 - 3 are accessible after the 7th Badge, so that would make me the 8th gym leader.
An experienced coordinator in my home region of Sinnoh and superfan of Fantina, I wanted to bring the joy of contests to Kanto too. As such, on arrival at the Sevii Islands, I took over the disused Game Corner on Two Island, which had been closed down due to anti-gambling laws, and converted it into Kanto's first and only Super Contest Hall (no contest pass required). A teenage Lostelle (who sometimes appears in the contests with her Hypno) and her elderly Dad run it. I then set up my gym on Three Island.
However, My gym cannot be challenged straightaway--instead, they find Three Island is overrun with the members of an upstart, local gang, who've stolen almost all the townsfolk's Pokemon! To restore peace and continue their Gym challenge, the player must battle their way to the heart of the Berry Forest where the local gang's headquarters are. There they'll find me locked in a triple battle with three grunts who're, the boss' last line of defence, exhausted, nearly overcome after having battled my way there too. Whilst I occupy the grunts, the player is free to fight the boss. When they win, order restored, and I tell the player to find me at my gym when they're ready before leaving to escort the evil team leader to the nearest Officer Jenny. On my way out I give them a Poffin Case as thanks.
After the player beats me at my gym, I will sometimes compete alongside the player in the Master Ranks of any Super Contest category. My gym has no trainers and I can be challenged in the first room (its new). Aside from distributing the 8th badge, the Promise Badge (the design would be two hands holding, one white and one black) I would also give TM30, Shadow Ball, to those who defeat me. My badge would allow the user to use HM09, Dive (and fortunately there would be a new area beyond the Berry Forest that has been opened up recently, which allow access to some dive pools). It would also increase the contest stats/condition of a player's Pokemon by a small amount.
Post-game my gym would be much improved. I've now recruited 3 gym trainers, and added three more floors to my gym (under ground), but idk what the puzzle would be. Probably some sort of door warp puzzle. I will also have a postgame appearance at Viridian Forest; when the player arrive, I talk about the habitat loss caused by the deforestation that has occurred there, and how I came to quell the angry spirits of the Pokemon that perished there as a result, using a special ritual passed down thru the generations of my home region. I give the player an Odd Keystone, and suggest they might want to take it to Altering Cave before flying away on Zeus, my Drifblim. Once the player has connected with 32 people via wifi / internet, a Hallowed Tower will appear in Altering Cave, where they may release the spirits I trapped in the Odd Keystone as a Spiritomb and capture it.
My character model would be bald head with a flatcap, black beard, a loose, shiny pink jacket with gold sparkles over a white shirt, black three quarter jeans, sandals, and sunglasses. I'd also be holding a chocolate frappe and maybe a little jewelry, like an ear stud. My dialogue would be scattered with small amounts of Esperanto word replacements (hi ---> saluton, bye -----> adiaux, etc).
Yes, I am one of those disliked leaders with only three Pokemon to their name.
My gym is just an ordinary house. Well, except for the fact that it's deep inside the rather creepy woods near the village. All you have to do to reach my house is to walk through it! It has a bunch of possessed trainers who will chase you around the maze-like forest! Make sure you defeat all the normal picknickers within a certain turn limit, or they might catch up to you...