An idea or two.

This is mainly a suggestion for a side-quest, and a fakemon, to whomever it may concern.

My idea was that, in the wheat field area (forgot what route) after you unlock mega evolution, crop circles appear in the wheat. a new Npc will be standing around, and mention something along the lines of "maybe the aliens who made this will come back if they see their kind?" if you have an evolution of s51 (forgot it's name) and have it mega evolve in a fight next to a crop circle, after the fight, a bunch of s51's pick you up and carry you away. the screen blacks out, and you find yourself stranded on an island cut off from everywhere. a strange hooded figure appears and directs you below ground into a natural maze. you make your way through the maze which has strange messages written in the walls in unknown symbols. reach the end of the cave, you find a bunch of s51 restraining an unknown pokemon. the hooded person reveals them-self to be the man from the crop circles, who explains that the s51's have been looking for an exceptional strong one of their kind to help them with this threat. you agree to help, the s51's part, and you are assaulted immediately by the pokemon.
This mysterious pokemon is only a suggestion, but it's a pseudo legendary called Umbrus. It's a dark/steel type with the ability Sunset- it clears away the effects like sunny day or rain dance and replaces it with "sunset", which powers up dark type moves and has a chance to make any battling pokemon fall asleep at the start of their turn. It's backstory is kinda cool- it's part of a triad representing dawn, sunset, and midnight that lives on a far away continent. however, it was mysteriously swept away to tandor, and is slightly effected by the radiation. it's not a nuclear type, but it's radiating nuclear energy all the same, and the nucler energy was making it hostile.. the s51's were restraining it to prevent it from contaminating anything or harming anyone.

But currently the only way to get an S51 (other than the current Mystery Gift) is by doing a sidequest about crop circles in the wheat fields. The only difference is, the one behind it is the gym leader of Vinoville Town. Another thing is, after the melt down near Vinoville, there's no going back until the end of the game. It's one of the places where you will fight nuclear Pokemon.