More infomation released end of this month and middle of july. Hear is what pokemon are returning.

Bulbasaur, Ivysaur. Venusaur
Charmander, Charmeleon, Charizard
Squirrel, Wartortle, Blastoise
Caterpie, Metapod,Butterfree
Weedle, Kakuna, Beedrill
Rattatta, Raticate + Alolan Forms
Pikachu, Raichu + Alolan Form
Golden, Seaking
Nidoran (Female/Male)
Nidorina/Nidorino
Nidoqueen/Nidoking
Clefa, Clefairy, Clefable
Vulpix, Ninetales + Alola Form
Igglybuff, Jigglypuff, Wigglytuff
Zubat, Golbat, Crobat
Oddish, Gloom, Vileplume, Bellossom
Diglett, Dugtrio + Alola Form
Meowth, Persian + Alola Forms
Abra, Kadabra, Alakazam
Poliwag, Poliwhirl, Poliwrath, Politoed
Machop, Machoke, Machamp
Tentacool, Tentacruel
Geodude, Graveler, Golem + Alola Form
Slowpoke, Slowbro, Slowking
Magnemite, Magneton, Magnezone
Farfetch’d
Grimer, Muk + Alola Form
Shelder, Cloyster
Gastly, Haunter, Gengar
Onix, Steelix
Krabby, Kingler
Exeggcute, Exeggutor + Alola Form
Cubone, Marowak + Alola Form
Tyrogue Hitmonchan, Hitmonlee, Hitmontop
Rhyhorn, Rhydon, Rhyperior
Happiny, Chansey, Blissey
Kangshaskhan
Staryu, Starmie
Mime. Jr., Mr. Mime
Scyther, Scizor
Elekid, Electabuzz, Electivire
Magby, Magmar, Magmortar
Magikarp, Gyardos
Lapras
Ditto
Eevee, Vaporeon, Jolteon, Flareon, Espeon, Umbreon, Glaceon, Leafeon, Sylveon
Kabuto, Kabutops
Munchlax, Snorlax
Articuno. Zapdos. Moltres
Dratini, Dragonair, Dragonite
Mewtwo
Mew
Chikorita, Bayleef, Meganium
Cyndaquil, Quilava, Typhlosion
Totodile, Croconaw, Feraligatr
HootHoot, Noctowl
Spinarak, Ariados
Togepi, Togetic, Togekiss
Natu, Xatu
Mareep, Flaafy, Ampharos
Bellossom
Azurill, Marill, Azumarill
Bonsly, Sudowoodo
Sunkern, Sunflora
Yanma, Yanmega
Murkrow, Honchcrow
Unown
Gligar, Glicsor
Snubbull, Granbull
Qwilfish
Shuckle
Heracross
Sneasel, Weavile
Teddiursa. Ursaring
Swinub, Piloswine
Remoraid, Octillery
Delibird
Skarmory
Houndour, Houndoom
Phanphy, Donphan
Stantler
Smeargle
Raikou
Entei
Suicune
Larvitar
Pupitar
Tyranitar
Lugia
Ho-Oh