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Proper dungeons. I'm not talking simply getting away from hallways or just stumbling around a cave, although that would be part of it. Give us locations that have multiple paths that lead to different areas like mt coronet but also have optional secret areas that legitimately take effort to find like hidden behind tall grass or locked behind a puzzle that takes thought to solve beyond just shoving 1-3 rocks into holes and getting hints at the hidden locations from lore scattered throughout the world like talking to a bunch of npcs or in those bookshelves that currently often just say something along the lines of "It's full of hard to read books about ___" or "It's a cookbook full of recipes for pokemon treats" so it rewards exploration. Of course inside these dungeons would be mazes with higher leveled pokemon and some that aren't otherwise available in the surrounding areas that are very hard to catch but not legendaries serving as repeatable bosses, maybe with exclusive moves or hidden abilities or guaranteed good IVs in a certain stat, and the items you find inside are increasingly more useful like potions/berries>revives/pp restores>held items>evolution items>drops for winning the boss fight - typically battle facility stuff like ability capsules or special items like the enigma berry, rare drops would be either unique cosmetic accessories for your trainer like a belt made of shed kommo-o scales or shit like bottle caps. The way out leads back to main routes but in ways that make them clearly inaccessible from that spot like sliding down a very steep slope or getting dragged through a rough current into the ocean.