Well, that's the nutty thing no? As far as we know, there is no way to travel between Unova, Kalos, and the Pokemon Nation. How do you decide which ones appear? If we plan out our teams, we can at least have some filler trainers we can do other stuff with. Like, snag a Rhydon and find him a good home? Or snag Pokemon back for trainers whom ask/pay?
You don't have to plan either. You can go with what the RP gives you. But at least if you do plan it, we won't have people fighting over whom gets first dibs on a shadow Pokemon they want.