Description is a key point when it comes to creating a new region. In fact, I'd say that it's easily the most important point. Creating a whole new region isn't as simple as putting Pokemon in certain environments. You have to explain why your region has the Pokemon it does, how they got there, how they survive and thrive, etc., even if it's in little tidbits. If there are Fakemon in your story (which I'd be perfectly fine with, if they're well done), you have to describe exactly what they look like. Otherwise, you're going to have a lot of confused readers.
The environments themselves are also important. If we don't know what makes them unique from other regions, how can we connect to the region itself? Describe the landscape, the type of trees, if there are any trees, what kind of water there is, the temperature, if there's humidity or not, etc. Again, write as if the readers are complete newcomers to the Pokemon world, especially this region.
And even after the environments and Pokemon are complete, you still have to think of other things: the towns, the people, the specific cultures they have, and the specific league rules, if there are any. How have the towns adapted to the region? Are they mostly rural? How much urbanization has occured? How have the people adapted? What are their beliefs? What kind of jobs are out there aside from training? How have they used their Pokemon to help them out? What does the league allow and not allow? Do they enforce their rules and regulations? If so, how strictly? ...Or do the people not use Pokemon? Do they fear them? Do they hate Pokemon and have prejudiced thoughts when it comes to trainers? If so, why? Is there even a league?
Thoughts like that have to come up when you're writing down an original region. You are basically creating an entirely new world and dropping readers right in the middle of it. We don't know anything about this region, so you have to describe it for us.
If you can do that, this story will be very successful. I hope my advice has helped.
...So, yeah, basically what Jax said.
Warmonger - A fantasy fic where peasants fight the wars of kings and lords, where monsters lurk in the night, and where depravity is not only expected, but encouraged...