Man I hate to keep shooting you down, but yeah its a world-wide event. Our story just takes place in Hoenn because its ground zero for what happened. You could maybe restructure your story a bit, like say you're still from Unova. Maybe instead of a Gym leader, you are the son of a city leader? Not all of them need to be terrible people. They used planes in order to trade with other communities. Your father went missing after a trip to Hoenn. Your mom, now sole leader of the city, had many obligations and often wasn't able to give you as much attention as either of you liked. She gave you the rufflet and all that. I don't imagine Gym battles still being a thing, but you and your companions could have traveled all of Unova anyway, maybe with some other goal in mind, or just because. Motivation for coming to Hoenn could be the same: find out what happened to your dad.
Or maybe instead of your parent being a gym leader, they are a gladiator in a city-state. Others face off against him/ her just like an old gym leader, except bloodier haha. We can spit-ball some more ideas if you'd like too. Also, the plane thing can still work, some old technologies have been saved, but they just haven't progressed. You'd be flying the same planes that were made before Groudon, the ones that were managed to be saved. Being from Mistralton city would give you access to those.
Sorry about that, and hope this helps! Let me know if you have any questions or if theres something I can clarify for you