I don't think I need to suggest Ralts given how much others love Gallade and Gardevoir as much as I do. What I haven't seen though is suggestions for Sawk and Golurk. I loved Stephen's Sawk in the show and if I could have even some of that I'd be happy. Also Golurk has always been super cool to me! I named mine POWERHOUSE and every Golurk in the show had this immense presence for the little showtime they got and I would love to have on on my team, maybe even evolving it from Golett. Some more "niche" pokemon I don't hear much about but LOVE nonetheless are:
Tangrowth - big caring lugs Hitmonlee - love them spring kicks
Muk - Ash's Muk was awesome Steelix - So intimidating
Ninjask - to scratch that bug weeb itch i guess i have
Torterra -I've always wanted to have a picnic on their back, and that rock climb in the show was SWEET
Drapion - Gray could be mean but that Drapion was COOL
Electivire - Never has a smile scared me so Gliscor - I felt that struggle when Ash's learned to glide
Lilligant - #TopTierBotany Vanilluxe - I mean come on, it's ice cream
Galvantula - A bug type I think is cool Jellicent - I love the male mustache
Ferrothorn - SPIN MOVE Chesnaught - Makes me feel safe
Tyrantrum - Makes me feel safe in a don't you dare touch my trainer sort of way
Trevenant - Spooky done right Decidueye - I called mine Cocoa Butler
Lurantis - Botany Assistant Golisopod - Big Boi Bug
Palossand - I like Sand Castles
I know I can't hope for every Pokemon on my list, MUCH LESS every Pokemon in the series, but this is just a list of the ones I really enjoy using that I don't see a lot of other people asking for. I get that people are gonna ask for Pikachu, Charizard, Eevee, and a bunch of Legendaries (some of which are already in the game or confirmed to not be in the game). I get that some of these are from the most recent release and perhaps we won't see them, much like we haven't seen the new trainers (which I'm not particularly bummed about to be honest). However these Pokemon I love with a passion and being able to have any of them on my team, better yet have actual facetime with a la Pikachu in the current build, would make me love this game even more than I already do.