You know what, considering recent events, going to drop my two scents on this "Power Plant/Volcanion" discussion.
FUN FACT: Some recent discovered of the October 2024 Mega Leak is XY location concept artwork. If you're alright with seeing leaked info, I highly recommend looking them up, because
the pictures are GOREGEOUS! (There was
also this one not included in the imgur collection)
And, pertaining to this discussion, several of the pictures are of the different Power Plant facilities; it gets 5 pictures dedicated to it. And most aren't just a landscape shot of the facilities outside, it's a dynamic picture showing the type of energy it gathers.
Another lesson from the 2024 Leak: A lot of initial plans get changed. Pokemon, characters, story, lore. What we see in the final product is the tip of an iceberg of years of tweaking and adjustments of what was going to be in the final product.
Point is, yes, in the final game the locked doors in the Power Plant is just that: locked doors. People need to let go any notion there is anything behind them or it was like an event that never got released. No. It was decided before the game's release, when they were finishing up that location, that locked doors may as well be a paintings on rock outcroppings. It's purpose was to allude to the idea that, yes, the facilities you are seeing in the "distance" have a entrance leading to them and the power plant is an entire underground complex instead of just the few rooms the player had to concern themselves with. End of story.
... BUT just because that's the end of the story doesn't mean it can't be speculated at some point GF may have had higher aspirations for the location. Because, say as much as you want it's just a one-and-done set piece in the final game, a lot of attention was given to it. Aside from the concept art, they did go out of their way to create the other facilities in the "distance" and giving said facility a (locked) door. They especially didn't need to do the latter part, players could just assume the other facilities are either accessible via underground passages or have their own entrance more closer to them.
And then we have Volcanion. BUT WAIT! Volcanion isn't the only Mythical in XY. We also have Diancie and Hoopa. And know what else we have two of? Seemingly random places that, in a way, vaguely match the Mythicals.
pika pal already mentioned Reflection Cave (where you can find Carbink) having an isolated room off the beaten path (one where you need to use the mirror walls to even see the door to access it) which has light shining down; in the final games its just the location for Alakazite. Then on Route 22 there's the Chamber of Emptiness, a small cave that just has the Spooky Plate and Banettite; the map description reading "A mysterious void. It is said that nothing can exist within it".
It's not hard to see that people speculated that maybe originally, instead of just being given the Mythical, we would be given an item to activate an event when we went to the right location (obviously, if this was the original plan, it was scrapped way before any item or in-game script could be implemented). For Diancie it would be the Reflection Cave, for Hoopa it would be the Chamber of Emptiness, and for Volcanion it would be the locked doors of the Power Plant.
"But that would just be one of the Locked Doors, and doesn't make sense for Volcanion to reside in the Power Plant". Well, going to the location would just be activating the event, there may have been more steps to the event before we could encounter the Mythical and catch it. Some thoughts on what they could have done (or at least what I would have done):
- Like for Diancie maybe that small event in the Pokemon Center which triggers when you get it is remnant of a bigger one. They'd appear after you claim the Key Item and tell you to go to Reflection Cave to find Diancie, you'd then have to battle through butlers and maids who want to capture Diancie for their master, and finally after catching Diancie we get the scene of their master telling the head servents to knock it off and just seeing Diancie in-person is enough for him.
- For Hoopa, whose gimmick is creating portals to any place in the world, I can see you having to chase it all around Kalos. It appears in the Chamber of Emptiness drawn by the Key Item you got, then creates a portal it goes through. While you can't go through the portal, you can see an image of someplace in Kalos. You go there and Hoopa sneaks up onto you (maybe summons a Pokemon to battle you) before jumping into another ring. Rinse and repeat until finally getting to the location where you can capture Hoopa.
- As for Volcanion, well who says Volcanion is at the Power Plant by its own choice? Maybe that event story would have been about the Power Plant having a malfunction, you using the Key Item to open one of the locked doors surprising the workers behind it, and deciding they need a strong trainer's help tell you that when the Power Plant was first made they captured Volcanion to help power it at the start. However Volcanion has suddenly gone berserk and is causing the Power Plant to malfunction. However the problem is they don't have the key card to access the facility Volcanion is in, but maybe their boss does. You go deeper into that part of the Power Plant, navigating a puzzle relating to its gimmick, and find the boss who, while he doesn't have the right key card, he has another facilities key card. You use that card to open another of the locked doors, navigate its puzzle, and find the person who does have the right key card. Finally you can enter the facility with Volcanion and capture it, saving the Power Plant who let you keep Volcanion as they feel guilty for exploiting it longer than they should have.
It would have made for a fun events as we'd also learn more about the Mythicals. But, considering some other things we learned about XY's development from the leak, already it seemed like GF bit of more than they could chew and had to pull back considerably. And maybe these speculated events were one of the things lost. It just seems odd to have these locations fitting for these Mythicals to not really have anything to do with them. The radiant room in the Cavern of Reflection and the whole Chamber of Emptiness just house a Mega Stone (and Arceus plate for CoE). The Power Plant having 3 interactable locked doors that are implied to lead to the other facilities, but in the game itself they're just decorations with no locations connected to them.
It's fun to speculate as I just showed, but you're right people need to let go. XY is made, there's no DLC ever coming for it. But while that's the final nail in the coffin there, we've got Legends Z-A coming.
