>>38569972I'd make a 100 floor dungeon. You'd start with 1 Pokemon at level 5, and get 100 or so Ultra Balls for your descent down. As you go deeper, the level of wild pokemon increase, as well as movepool and enemy AI. At the start of each floor, there are ancient rune markings that heal your party (similar to the heal spot in the Pokemon Tower in Lavender Town). Each floor would have a "Guardian" Pokemon, that acts like a Boss monster or Totem Pokemon (For example, floor 10 might have a Guardian Nosepass). It would have guaranteed good IVs, and even have EVs as the floors get lower. Every floor would also have a bit of lore about the history of the Pokemon world, but starting around the 20th floor, it switches from the human language to Unknown. Once you reach to about floor 50, the environments completely change on each floor, from a grassland environment, to a lava covered wasteland.
