Underused and unused concepts in generations

Kanto:

-Legendary Birds and their connection to Lugia: Just what is it?

-Ruby and Sapphire items: Representing Passion and Honesty respectively, these two are connected to Groudon and Kyogre somehow, but how, is unknown.

Johto:

-Falkner's dad: Apparently is dead in B2W2, but in HGSS it is ambiguous.

-Legendary Beasts before the burning of Brass Tower: What they were?

Hoenn:

-Jade Orb and worship of Kyogre/Groudon: One could expect Primal Rayquaza and Mega Kyogre/Groudon to come about these.

Sinnoh:

-Red Chain: Used by TG, but not player. Exists as an unused item.

-Arceus and legendaries stats: They look pathetic compared to current Megas and Primals.

-Solaceon Ruins: Outside of Unown and Maniac Tunnel, it has little point.

-Toy Robot Rotom: One of its available formes, but it is not mentioned in anywhere.

-Fog: Its text exists in all generations after Sinnoh, but itself has not shown up.

-Buck and Flint's relationship: In official Quiz books its confirmed that they are siblings, but this has not reflected to games canon.

Unova:

-God Stone: Unused item for Kyurem. Since Kyurem is normally available, this thing is not used, but it can be reused.

-Ultimate Dragon: Constantly alluded, but not revealed.

-Fog in Unova. Texts exist, but they are not used.

-Thunderstorm and windstorm: Special weathers for Thundurus and Tornadus. Used for cosmetic purposes only.

-Plasma sages: With the exception of Rood and Zinzolin you cannot battle them.

-Village of Dragons: In games, it doesn't exist in Unova, so where it is?

-Keldeo's parents and cause of the fire: Never confirmed, so it is left hanging.

Kalos:

-Southern Kalos: Alluded to by Volcanion and Kiloude City, and known to exist thanks to real-life map of France, Southern Kalos remains an unused concept.

-Malva's TF connection: Obviously implied and used in Essentia storyline, Malva's role in the actual plot has not been used.

-Kalos Power Plants: The other plants remain closed.

-AZ's Floette and Thousand moves: Still not used.

-Team Flare scientists and E4 Megas: The latter is not used, and former is underused. Their storylines can be expanded and new Megas can be given.

Others:

-Spirit World: Constantly referenced, but there is no direct entrance to it yet. Distortion World isn't really it, though it seems to be connected regardless.

-Inverse Battles: Inver's abilities come either from his Psychic abilities(in XY) or a machine he developed(in ORAS), so you would expect this to be something.

-Guyana: Homeland of Mew, and in Animé, Lapidarian Highlands,  is never seen outside of Animé shots.