Nature Power becomes: Nature Power becomes always a Special Attack with 70<BP<110.
While it is true that Nature Power is always a Special Attack, the rest of that is inaccurate. Nature Power's minimum base power is actually 65, seen with Mud Bomb. On top of that, it actually does hit a base power of 110 exactly in sea, where it turns into Hydro Pump. So that should be, "Nature Power always becomes a Special Attack with 65≤BP≤110."
Now for my submissions:
Environment name: Another Dimension
Weather effects: None.
Nature Power becomes: Hyperspace Hole.
Secret Power has the animation of: Confusion, and may Disable the foe's last-used move.
Camouflage changes the user's type to: ???/none (or if not possible, Psychic)
Other effects: Trick Room and Wonder Room are always active; using any of these moves instead deactivates the effects for the duration the move would normally be active. That means all your physical walls are now special walls and vice-versa, and everything that was slow is now fast. Psychic-type moves, Nature Power, and Secret Power have their priority increased by 1.
Flavour: An alternate dimension where nothing makes sense and everything is inverted. Hyperspace Hole represents dimensional travel. Psychic-type moves have priority because this dimension is especially vulnerable to space-manipulating effects like telekinesis? I dunno, basically because magic and because it's a Psychic-themed environment. Secret Power is basically an honorary Psychic-type move, so it gets that benefit as well.
Environment name: Crystal Cave
Weather effects: None.
Nature Power becomes: Power Gem.
Secret Power has the animation of: Dazzling Gleam, and may cause flinching.
Camouflage changes the user's type to: Rock.
Other effects: Grounded Pokémon cannot have their stats lowered by any means other than Clear Smog or Haze. Rock-type Pokémon have the effect of Serene Grace in addition to their regular ability. Rock-type moves, Nature Power, and Secret Power have their priority increased by 1. The move Flash reduces the target's Accuracy by 2 stages instead of 1 (though it is ineffective against grounded Pokémon for reasons stated above). Pokémon with Illuminate reduce the Accuracy of non-grounded foes by 1 stage on switch-in.
Flavour: The fantastical nature of the cavern brings out the hidden beauty in Rock-types, granting them the effects of Serene Grace. Grounded Pokémon cannot have their stats lowered because the clarity of the crystals can be likened to Clear Body. Flash's intensity is increased because the light reflects and refracts through all the crystals in the cavern; this is also why the ability Illuminate causes an accuracy drop. Rock-type moves have priority for some magical reason based on the beauty of crystals, I dunno but it makes more sense than a damage boost. Secret Power gains priority due to being an honorary Rock-type move. Secret Power has the animation of Dazzling Gleam because it is powered by the refracted light from the crystals, and it causes flinching because foes can't bear to look at the light (and because an accuracy nerf effect would be useless here).
Environment name: Graveyard
Weather effects: None.
Nature Power becomes: Shadow Ball.
Secret Power has the animation of: Shadow Sneak, and may cause flinching.
Camouflage changes the user's type to: Ghost.
Other effects: Grounded Pokémon have a Life Orb effect in addition to their item; this effect stacks with an actual Life Orb if the Pokémon is holding one, including stacking the recoil. The effect of Heal Block is always active for both sides of the field. Ghost-type Pokémon are vulnerable to trapping moves and abilities.
Flavour: The spirits of the dead seek to add to their ranks, so the Graveyard is a place of desolation and pain. All Pokémon have their life energy drained to fuel their attacks by the will of the dead, and healing is impossible due to the tormenting of malevolent spirits. The strong spiritual ties that Ghost-types have to a place such as this allows them to be trapped here, even when that would normally be impossible. Secret Power has the animation of Shadow Sneak with a flinch chance because that is what it does in Mt. Pyre in ORAS (though I personally liked my original idea of Ominous Wind's animation with a Curse chance better).
Competitively, Pokémon with Oblivious are very powerful here, as Heal Block is considered a mental effect. Pokémon with the ability Oblivious are unaffected by those, so they are able to heal as normal.