I'm trying to get some storyline ideas/events at the moment. Here's the base so far:
The region of Ferrum is full of lush, green scenery. Each year, a SHAYMIN comes to restore the region's beauty after each harsh, cold winter. But, an unexpected team named TEAM ASCUS comes and steals the SHAYMIN, leaving the region in peril. As you journey, you will witness this catastrophe, and vow to save the region of Ferrum...
Here's the untold part:
So Team Ascus needed Shaymin (The creator of ferrum) to create a machine that enables going between the reversed world and the real world. The only flaw was that if shaymin doesn't replenish the region of ferrum after each harsh winter, ferrum will slowly merge into the reversed world. You first meet them in Trident City, as they attempt to steal a pokemon from someone, and then they reveal their identity. That's pretty much all I've come up with