Meh, just some thoughts I thought I'd share. I'm obviously not counting characters already in the game who I don't have yet(cough, cough, Marley and Thorton, cough), just characters that literally aren't in the game.

1, 2, and 3. Elaine, Chase, and Trace

Please Pokémon Masters, give me my children. I don't care if they completely ruin my headcanons for Chase and Elaine, I just want them playable. I'm not sure if they'll let more than one person have a Pikachu, but they gotta. For my sake. Trace can have Cubone/Marowak so he's different from Blue.

4. Sabrina

Sabrina was confirmed in the reveal trailer, but she still isn't in the game. I want to make a childhood obsession team with her, Marley, and Misty dammit! Also I want to hear her voice. Hopefully they don't mess up like they did with my goth girlfriend Marley.

5, 6, 7, 8, 9, and 10. Bea, Allister, Nessa, Milo, Bede, and Marnie

I know how gatcha games like this work, alright? When the main series makes a shiny new entry, the mobile game promotes it. So I think it's really likely that Sword and Shield characters will get in. My most wanted is Bea because she's my favorite. But I honestly love all of them. I added two of the rivals to this mostly gym leader entry because I could, don't shame me.

11 and 12. Greta and Anabel

The queens of my heart should be in this cash grab to bring some sanity. I thought they wouldn't have a chance, but when Noland was in the game everything changed. I'm reeeeally hoping for Emerald!Anabel, but Su/Mo!Anabel would be cool too. I just want Greta and Anabel to be together again.

13. Mira

That little girl with the wacky hair who was lost in a cave until Lucas/Dawn found her, who is actually a strong trainer who specializes in Special Attack would be a perfect addition to the roster. She also speaks in third person in the games, so I want to see if Pokémon Masters would keep that or be cowards and omit it like the anime who aged her up.

14 and 15. Emmet and Ingo

Please, give us the Subway Bosses fan service we deserve. Pokémon Masters has shown that twins can be separate, so I can totally imagine Emmet and Ingo in the game. I don't know what Pokémon they'd use, but that's more on me being too lazy to check Bulbapedia. Also more gen 5 stuff would be very, very nice!

16 and 17. Calem and Serena

I obviously want all of the protagonists at some point, but for some reason I really want to see Calem and Serena. If they go the coward route and make Serena the rival like they did with May I'll scream.

18. R/S/E May 

This might be the only way to get protagonist!May, which is my headcanon. Also May has had consistently good designs in all of her incarnations. You know what they say, more Hoenn more better!

19. Jasmine

I was surprised to see that Jasmine was not in the base game. I really like her and want to protect her from nasty Houndooms. If she was in the game, I could do that. Also, she's enough of a "waifu" to make money for Pokémon Masters. She's also strong.

20. Lana

Lana is unironically best Su/Mo girl. She is super funny and I feel like she could have some great lines, both written and spoken. I would put her on my team over pretty much every other Water-type trainer. No matter what Pokémon they might choose for her, I'm sure it would fit and have useful skills. If not, I would force it anyway.

That's my list. It's not that late where I am, but I'm super tired. Leave opinions, if you'd like!