If you don't have a lot of information on the "canon world", you can take the time to research for your writing. If you're following the anime canon, for example, read episode guides online. There are quite a few websites out there that are full of information on Pokemon.
If you're asking for ideas about what to write, then you've come to the wrong place, pretty much. We're all exceptionally snarky when it comes to helping writers get ideas, and you'll wind up with a story no one will want to read. Also, you'll have a story idea that you don't want to write. And if you don't want to write the story, then it's not going to go anywhere.
To come up with ideas for stories that you would want to write, brainstorm a bit. Think about the Pokemon world, and if there's some part of it that you want to explore. Or if you want to take your own personal views on a story that others have written, like a different take on the original trainer formula.
Since you're asking about Pokemon Ranger fics, I have to ask if you've played either one of the games, since that is mostly what the stories I've seen are based on. If you want to do write the story of the games into a fic form, then you need to take the time to put your own personal touch into the story to make it so that readers are interested.
I really haven't seen a lot of Ranger fics around here recently. I know that there are a few elsewhere that I've seen, but never read.
