Legends:
After x and y (about 50 years)
Quasar is shadier than they seem.
Zygard is captured and ecosystem starts crazy.
Zeraora steps in to find zygard and causes quasar to set their sites on it.
Quasar already controls Volcanion in the power plants are trying to obtain control of all power forming beings in Kalos to improve Lumiose.
Unknowing of their actual plans you join with good intent. You help quasar expand the city after finding 5 “power” generating pokemon.
As game continues you find out their real plans and try to stop them by teaming up with zeraora (who is the last “boss”) and find zygard
Mega evolution and trainer-pokemon bonds are merging together so trainers have no need of their stones (think ashgreninja)
Mega evolution becomes out of no where because zygard missing causing new megas: flygon, greninja, chestnaught, delfox, starters, Luxray, Roserade, Scolipede, Scrafty, zygard, yveltl, xernes, zeraora, volcanion, noivern
New forms: starters (water, psychic) (fire, rock) (grass, dragon), noivern, florgies and more if finished
Additional evolution: gogoat (grass electric), noivern (dragon, electric), lycanroc (form 1: rock, fire; form 2: rock, electric; form 3: rock, water)

Gen 10:
Greek region with 10 islands
8 islands have gyms
Each island has a theme
Box legendaries are a dragon, flying type and ghost, water type
These two legendaries are based off of Zeus and Poseidon
Third legend is Fairy and Fire. Based off Hades
Starters are based off of a minotaur, pegasus and gryphon
Has new pokemon similar to eevee but bird. Has all evolutions eevee does not have. Evolves based off moves it knows
Can travel the region in any order by boat or glider depending on version
Once at island, you have to complete it
Story progresses in any gym order
Minigames are back for gyms but better (like poken tournament, mystery dungeon and other sideline games)

Ps:
Taking control of pokemon is major in both games. Similar to blueberry dlc and mystery dungeon