Metagame Premise: Familymons.
It's basically STABmons, but instead of being able to use any moves sharing your typing, you can use any moves in the learnests of Pokemon sharing the same egg group.
I explain: If i want to use Pinsir-Mega, i check its egg group, it says Bug, that means Pinsir-Mega can now use all the moves from the Bug egg group like Megahorn from Heracross or Stealth Rock from Gliscor! A Pokemon cannot borrow more than 2 moves this way.
FAQ: Q: What about Pokemon that have several egg groups?
A: Those Pokemon can choose moves from Pokemon from either of the egg group they share with with still the limit of two moves maximum.
Q: What about the Undiscovered Egg Group?
A: For the few Pokemon in this egg group that aren't banned from OU, they can use all moves coming from Pokemon in the Undiscovered Egg Group. That means that Landorus can now use Thousand Arrows thanks to Zygarde or Dragon Ascent thanks to Rayquaza.
Potential Banlist: OU Banlist, Chatter, Geomancy, Belly Drum (maybe), Jirachi, Kartana, Xurkitree (maybe) and taking moves from Smeargle.
Potential Threats:
Landorus-Therian@Rocky Helmet, Leftovers
Ability: Intimidate
Spread: 244 HP 252 DEF 8 SpD 8 Spe
Impish Nature
Roost/Recover
Thousand Arrows
Stealth Rock
U turn
FakeSpeed might make a comeback thanks to several egg groups getting the combo and STAB on it. Here comes Landorus-T that will, like in STABmons, be very useful as a check to them. Now it gets reliable recovery in the form of Roost or Recover, and the precious Thousand Arrows which let it not worry about airborne Pokemon that would switch with impunity otherwise. Rocky Helmet punishes FakeSpeed, U turn, and overall contact moves whereas Leftovers gives more passive longevity. Pretty good defensive Pokemon all around.
Diggersby@Normalium Z/Sitrus Berry
Ability: Huge Power
Spread: 4 HP 252 Atk 252 Spe
Adamant Nature
Belly Drum
Extreme Speed
Earthquake
Knock Off
OMG BellySpeed Diggersby is back! Thanks to sharing the same egg group with Linoone, it gets both Belly Drum and Extreme Speed, if the bunny manages somehow to setup (and thx to Normalium Z it will always kinda) better bring a fast Pokemon that resist or is immune to Normal otherwise you'll get swept pretty easily. EQ and Knock Off cover steels and Ghost types respectively. But really, why aren't you just clicking Extreme Speed? Probably a ban of Belly Drum or Diggersby itself.
Aerodactyl@Choice Band
Ability: Rock Head
Spread: 252 atk, 252 Speed
Jolly Nature
Head Smash/Stone Edge
Brave Bird
Flare Blitz/Fire Fang
Earthquake
Thanks to being in the Flying egg group, Aerodactyl gains new tools to abuse Rock Head. in the form of its STAB Head Smash and Brave Bird, it can deal pretty serious damage to almost anything when coupled with Choice Band. Flare Blitz can be used if Stone Edge is prefered over Head Smash to still hit airborne Steel types very hard, it also hits Ferrothorn harder than Brave Bird and Magearna super effectively. Earthquake is here mostly filler, as Rock+Flying+Fire already hits everything pretty hard, but it although still hits Magearna hard if Flare Blitz isn't used.
Serperior
Life Orb/Assault Vest
Ability: Contrary
Spread: 252 SpA, 252 Speed
Timid Nature
Leaf Storm
Overheat
Draco Meteor
Giga Drain/Hidden Power Ground
Serperior is one of the few examples that really benefits from being part of two different egg groups, as it can borrow Overheat from Infernape and Draco Meteor from Exeggutor-Alola. Grass+Fire+Dragon is only walled by Heatran so far, and with Hidden Power Ground, the threat is covered. Of course Chansey and Unaware Clefable are still a problem for Serperior, so it will need teammates to take them out. But beware of this majestic snake, as it could sweep your team entirely if you let it boost.
Questions for the Community:
Should i ban Pokemon from the Undiscovered Egg Group from borrowing moves from each other? And then not ban Jirachi, Kartana or Xurkitree? I have the feeling it would not make the meta interesting, by removing Landorus Therian's access to recovery to stop FakeSpeed for instance.
About moves illegalities with hidden abilities or event moves on some Pokemon, should they apply if you borrow one of those moves? Or not?.
And Voila.