...this is the premise I considered sending to Japan...
Ash, Misty (yes, Misty and not May), and Brock are somewhere in Hoenn when they are summoned by Professors Birch, Elm, Oak, Ivy, Westwood, etc. to the Indigo Plateau for an emergency meeting of high-level pokemon trainers. There has been a threat against various areas, including Kanto and Johto, with a mysterious man threatening to singlehandedly take over these prefectures using his destructive pokemon...he won't reveal what pokemon he has, so search parties are made from strong trainers to find him...of course Ash's party eventually finds this man after meeting up with other parties who were absolutely ravaged by his single pokemon. After tracking him by his path of destruction through many prefectures, and battles and obstacles created by some of his minions, they meet this evil man and must battle his legendary pokemon or risk subjugation...that mighty pokemon, mightier than all others, was then revealed to be DUNSPARCE!
(and the plot goes on and on from there)