(I spent a half hour on this, please don't let it flop)
Red: Charizard, Pikachu, Lapras, Snorlax, Espeon, and Mewtwo
Reasoning: Red has a Charizard and Snorlax in Masters, so they're immediately on his team. He's also the protagonist for pokemon yellow, so the pikachu is there also. The Lapras and Espeon were on his teams in the past (as well as being a gift pokemon, and him having one in the anime), so that works, and Red cannonically catches Mewtwo in the games, though he never ends up using it.
Ethan: Typhlosion (Or Feraligator for Kris, and Meganium for Lyra), Azumarill, Togekiss, Donphan, Girafarig, and Ho-oh (Perferably not, since Silver has Ho-oh)/Lugia/ or Suicune.
Reasoning: Each of the Johto protags have a perfered starter in Masters, Ethan has a Marill as the not protag, and uses a Donphan in the Zoroark: Master of Illusions movie, uses a girafarig in the anime, Togekiss is a gift pokemon, and Ho-oh/Lugia/Suicune depending on which game you played.
May: Swampert (or Sceptile for Brenden) Wailord, Breloom, Ludicolo, Raichu, and Groudon/ Kyogre.
Reasoning: A Wailmer, and Shroomish are used on the rivals team in the battle underneath the Cycling Road, Brenden and May have Sceptile and Swampert in Masters, They have a Ludicolo in emerald, raichu is used in the after credits battle for ORAS, and, well, you know the drill for Groudon and Kyogre.
Dawn: Torterra (Infernape for Lucas), Clefable, Alakazam, Magmortar, Lucario, and Dialga/Palkia/Girantina.
Reasoning: Again, Pokemon Masters, and Lucas/Dawn use Clefairy and Kadabra in the multi-battles against the bob-headed space people, Lucas has a Magmortar in the anime, you get a riolu egg in Iron island, and the legends are the same deal as always.
Hilbert: Samorott (Emboar for Hilda), Simisage (Simipoar for Hilda), Volcorona, Archeops, Beartic, and Zekrom/ Resherim.
Reasoning: Master starters yatta yatta yatta. Monke from the dream yard, egg volcorona, most subway used subway sets, and legendaries.
Rosa: Serperior (Emboar for Nate), Simipoar (Sage for Nate), Lucario, Arcanine, Zoroark, Sawsbuck.
Reasoning: Masters, Dream-yard, Lucario and Arcanine are used by Nate in the anime, Zoroark and Sawsbuck are given as gift pokemon as their base starters.
Serena: Delphox (Chesnaught for Calem), Blastoise (Charizard for Calem), Meowstic, Absol, Lucario, and Xerneas/YAVULTAL.
Reasoning: Sereana has a delphox in masters, Calem has a Chesnaught in Generations, Calem has a Charizard, so I gave Blastoise to Serena, Meowstic, and Absol are used on their rival teams, Gift Lucario, and Megas in specific games.
Elio: Primarina (Decidueye for Seline), Toucanon, Alolan Raichu, Alolan Ninetails, Dusk Lycanroc, and Tsareena. (Lillie gets nebby)
Reasoning: Official art and Masters, there's not a lot to go off of.
(Skipping over Let's go)
Victor: Cinderace (Rillaboom for Gloria), Urshifu, Calirex, Toxtricity, Eternatus, and The good boiz.
Reasoning: Events of the games, Again, not alot to go off of.