I have seen people mention that there are 3 power plants that can't be accessed, but there are actually 4, bringing the total to 5.
3 on the right of the Lumiose Badlands, and 2 on the left.
The one we visit is the top left facility and collects microwaves generated from solar energy by a facility in space.
The bottom left seems to be a large multi-floor hydroponics plant, probably creating fuel from biomass.
On the right are from top to bottom:
A raised platform with domes, not sure what this is, but it could be an anaerobic digestion plant. A facility that seems to be surrounded by volcanic vents, so probably using geothermal energy. Wind turbines
It seems as though a lot of thought and design went into these, but nothing happens with them...
One theory I read was that they related to a legendary trio, but that was based on there only being 3, not 4 inaccessible facilities.
What if there something inside besides legendaries?
3 new legends are known so for sure 3 of them will be in each room,and one or two of the rooms thats left over will probably have held items for those legends to boost their ability or something i can be wrong but its just a guess :D
One of the event pokemon or a new pokemon they have planned to release from that unknown region might actually change form. And if so then perhaps these new rooms are for them to change form? I could see these as being form changing rooms a lot easier than I could to them each holding a legendary pokemon.
I've got a quote that embodies you perfectly, but it's seventy-three posts long, has a few massive flowcharts, and lots of Xion-KMA to Me