How about Snowy Peak (Very tall mountain, similar to mt. coronet) having absol, mamoswine, cetitan etc. 
Flower Forest / Field with all those fairy and grass mons like whimsicott, ribombee, florges, comfey etc.
River having typcial river animals like psyduck, bibarel, magikarp maybe even surskit.
Maybe even rare biomes like a Dragon Layer having dragons like haxorus, druddigon, dragonair, tyrunt etc.
Mirror Cave from XY with psychic mons
There are so many biome ideas popping into my mind.
Canyon with Crustle, Lycanroc, Klawf, Dugtrio, Sandslash
Misty Forest from SwSh
Coral Reef (Maybe taking inspiration from New Pokemon Snap)
Bamboo Forest from SV
Abandoned City with Rotom, Klefki, Garbodor
Meteorite Crater with Minior, Clefable, Elgyum etc.
Sky Islands with Shaymin Sky Form as a potential Boss as well as regular mons such as Jumpluff, Tropius, Vivillon