Really the only people I would want to battle from the games who I couldn't were the Professors of each region, since they all have a major theme they could've based their teams on? I'll list a few examples below:
1) Professor Oak -
Oak's whole gig is to complete the original Kanto Pokedex, and seemingly was designed to be a final challenge for the player in the original Red/Blue games. Whilst that team could've worked here too, I decided to go with this one instead because it fit a bit better to me. Nidorino was described as his "ace" in Pokemon Masters I believe, and mirrors the rivalry with Agatha and the opening to the games (Nidorino vs. Gengar). I gave him the traditional F/W/G core with the starter you didn't pick and then the other two types, but I couldn't think of a Grass type he'd use (Vileplume maybe) so I figured I'd place Venusaur here and showcase Arcanine and Lapras a kind-of mix between Red and Blue's canonical teams. Aerodactyl and Snorlax are also a Red/Blue team mix, both are powerful Pokemon and I think Snorlax's presence in the region and the uniqueness of Aerodactyl means Oak would use them.
2) Professor Elm -
So according to Bulbapedia Elm is interested in Pokemon Abilities? I did not see that as a thing because they didn't exist in Gen 2 I don't think (they were Gen 3 right?). I assumed his focus was on Pokemon Eggs and breeding since he gives you the Togepi egg and an Everstone later on, so I went with that theme. And the theme is... Baby Pokemon that evolved? Togetic seemed like a given since he has a small connection to Togepi (maybe Mr Pokemon gave him another Togepi egg once you'd all figured out what it was). Raichu because Pichu is a Baby Pokemon and Pikachu is the mascot, Hitmontop is the only other new Pokemon (from the generation) which evolves from a Baby Pokemon and the last 3 gained Baby Pokemon pre-evolutions too. Wigglytuff and Clefable didn't make the cut because... I dunno, type overlap. Maybe replace Jynx with Wigglytuff, that could work.
3) Professor Rowan -
Rowan specialises in Pokemon evolution, so I slapped together a team which evolved in wacky ways, basically. Leafeon over Glaceon because Eterna City is closer than Snowpoint City and I think Grass suits him more. Gliscor over Weavile because Cyrus used the latter and whilst Bertha does use Gliscor in Platinum it's so minor I feel like it's acceptable for Rowan to use it instead. Magnezone also has a location-based evolution and it has more nostalgia than Probopass (as in, it's pre-evolution isn't from Gen 1), Mamoswine and Ambipom are Pokemon who evolve when they learn a new move and Wormadam evolves into different forms when in a different place (I picked Trash because he works in a town/city). Alternatives would've been Lickilicky or Tangrowth over Ambipom (with Glaceon over Leafeon if the latter is chosen) and Mothim and Vespiquen used over Magnezone and Mamoswine maybe?
4) Professor Magnolia -
Magnolia specialises in Dynamax, not Gigantamax, but I feel like her team would consist of Pokemon which can do so anyway! And yeah, I gave her Snorlax as well because... nothing else really fit. Alcremie maybe? I also went with Butterfree over Orbeetle because I just had a gut feeling it fit her better. Boltund cannot GMax but I like to think she has one and Sonia has a Yamper as a homage to it, but that's just me. Corviknight and Drednaw because they're found near her house and she has a Rookidee on her cane (I think?) - if not, replace Corviknight with something else. I figured she'd use the same starter Leon picked from your original trio (I'm #sobblesquad so she gets Rillaboom) and would potentially GMax it since Leon won't (you and Hop cover using the starter as a GMax ace) - realistically, I'd have her whole team GMax and it's randomly decided which one it'll be each battle.
