Ideas on future mascot legendaries

With the advent of evolving legends, we will likely have breeding legends in the future as well. So;

-Something like the Cosmog-line, but only has two stages.

-Something like a Legendary Simi trio, where all legendaries are related to each other and can breed with each other, while remaining separate lines in the dex.

-Something like East-West Gastrodon, where each Legendary is just a different form that is retained through birth.

-Something like Zangoose and Seviper where the two sides are opposites of each other but can breed with each other.

-Something like Skarmory and Mantine where two sides are differently distributed stat-wise, but share most of the stuff despite being biologically unrelated.

-A case where the legendary is related to a common Pokémon somehow, either requiring it to trigger its appearance/evolution, being its evolution or form, or being related to it in some other fashion.

-Reverse Nebby, here you own the thing since start, but cannot unlock its power until end-game.



Edit (09.06.2017):

Currently third legendaries look like this:

Gen I: N/A
Gen II: N/A
Gen III: 680, Rayquaza. Higher BST than Groudon/Kyogre(670). Has a Mega without item(780)
Gen IV: 680, Giratina. Same BST as others. Has an alt form with item.
Gen V: 660, Kyurem. Lower BST than others(680). Has two fusions with Key item.
Gen VI: 600, Zygarde. Lower BST than others(680). Has two forms, one weaker(496), other achieved through ability in-battle.
Gen VII: 600, Necrozma. Lower BST than others(680). Used for fusions for Legendaries.

So, logically:

Gen 9: 580
Gen 10: 520
Gen 11: 520
Gen 12: 500
Gen 13: 440
Gen 14: 440
Gen 15: 420
Gen 16: 360
Gen 17: 360
Gen 18: 340
Gen 19: 280
Gen 20: 280
Gen 21: 260
Gen 22: 200
Gen 23: 200
Gen 24: 180
Gen 25: 120
Gen 26: 120
Gen 27: 100
Gen 28: 40
Gen 29: 40
Gen 30: 20
Other brackets: 570, 590, 610, 620, 640, 650, 690, 710, 720, 730, 740, 750, 760,