The distortions would spawn a bunch of Pokemon. As for the Pokemon spawned inside the distortion, there could be a pool of Pokemon specifically for Space-time Distortions including Pokemon like Porygon2 as you mentioned. In addition, any "Ultra Rare" spawns for the biome that the distortion is in would become just as common. It would basically be an explosion of rare Pokemon.
I also recall space-time distortions creating a bunch of items, so perhaps a bunch of invisible items can appear on the ground in the distortion.
I agree with the space-time distortion concept. It would be a nice inclusion.