My own opnion for Go's future achievements:
First canine: A-Ninetales (As well as his third ice type) or Rockruff (Since he's only catching it to bring it back to Alola, unless they pull a Suicune and Absol on it)
Second Steel type: Klink or Durant(In case he doesn't catch Klink in the Labyrinth episode)
First Pink mon: G-Slowpoke or Snubbull.
First equine: K-Ponyta(Most of his catches a Kanto mons)
First Pika Clone: Pachirisu(Due to the remakes)
First Bovine: Tauros
First Caprinae: Mareep
Third Dark type: Murkrow or A-Rattata(He has both regional variants of Exeggutor, so why not?)
Second Amphibian: Wooper or Tympole
First Ursidae: Teddiursa
As far as firsts go:
Pseudo - Goomey/ Dreepy
Mythical - Darkrai (which has appeared in allot of merch and could be a PM mission), or Shaymin
Park/ team legendary - Kubfu
Pica clone - Dedenne
Goh has allot of Pokémon from the common types; water, poison, grass, flying, normal, bug, and normal (each 6+), but he does need more steel, dark, psychic, dragon, fairy, ice, electric, fire, fighting (3-), and rock (4).
Gen 4 Pokemon i think Goh will catch soon (August, September and October: Finneon/Lumineon, Shellos, evolve Misdreavus, Budew, Kricketot and Kricketune, whole Burmy line, Hippopotas
Other possibilities are; Combee, Starly, Bidoof, Drifloon, Munchlax, or Snover.
My wishlist for Goh's best from each region:
Kanto: Hitmonchan, Arcanine, Nidoqueen, Articuno
Johto: Scizor, Suicune, Heracross
Hoenn: Absol, Flygon, Latios
Sinnoh: Dusknoir, Magmortar, Drapion
Unova: Golurk, Zoroark, Volcarona
Kalos: Aurorus, Gogoat
Alola: Ninetales, Oranguru, Celesteela
Galar: Fully evolved starters and Arctozolt
As far as new captures/ capture-evolutions go:
Kanto: Ninetales, Alakazam, Lapras
Johto: Houndoom, Ampharos, Tyranitar
Hoenn: Gardevoir, Latias, Mylotic
Sinnoh: Froslass, Darkrai, Heatran
Unova: Zoroark, Volcarona, Genesect
Kalos: Aurorus, Tyrantrum, Aegislash
Alola: Ninetales, Silvally, Guzzlord
Galar: Thievul, Hatterene, Zapdos
Watch as the writers magically give Latias to Go while Ash gets Latios.
I always saw Latios as the more battle focused one so I would agree with that choice, though I don't think the Lati's/ Kubfu/ Toxel will go to the boys just because of 2 forms. It would create allot of overlap when prominent side characters like Tsurugi can be used to show off the other forms anyway.