I would love to see a Floral Jungle Scenery. I don't believe there has been a true jungle scenery in the main series games, which would have worked perfectly in the Ruby/Sapphire island setting. (My favorite overworld conceptually)
Something Reminscent of this:
Further, I would love to see a beautiful tropical/savanna region resembling African or South American coastal regions. The region would have pokemon that are bright, colorful, and exotic. There would certainly be several jungles, along with misty rivers/gorges, beaches, volcanoes (one that is hot and active the other that is frozen and dormant), treasure-filled caves, and perhaps a desert oasis with palm trees and inland streams. This sort of region would excellently compliment the more contemporary and industrial regions we have had.
As for pokemon. I'd love to see Toucans, Lemurs, flying foxes, Tenrecs (similar to porcupines except cuter), Peacocks, Geckos, DOLPHINS (I only hear this request like every single generation!), Gorilla, Deadly nightshade plant, Komodo Dragon, Frill Lizard, Coral Snake, Feathered Raptor, duck-billed dinosaur, Sabretooth Tiger, Cockatoo (Randomized Coloration), Antelope, Cuddlefish, Spanish Shawl Slugs, Fruit bat/fruit fly (a literal take on the concept!), Emerald Tree Boa/Anaconda, Fluorescent Comb Jellyfish, Vampire Squid, florescent betta fish, Actual Kangaroo, Actual Ant (with a queen that rivals Vespy), Okapi, Caracal/Lynx/Snow Leopard/Sphynx Cat, Culpeo (Racoon Wolf), Wombat, Marmoset, Latern Bug, Thorned Flower Dragon (of my imagination), Leeches, Angora Rabbit, and a Fluffy Moth (if anything I want the fluffy moth!).
Okay, wrote too much.