There are actaully ways that Yellow can be introduced in the SM Chapter like Green was in XY and Red & Blue were in ORAS
Yellow can get an Eevee from the Ecruteak Kimono Girls, since she was the bearer of the Rainbow and Silver Feathers (yes, those are feather, not wings) and alongside her Pikachu she could promote LGPE in the Alola story arc without making a chapter of it (I mean, there is Blacephalon in it, without a need for a USUM Chapter).
We can still see Necrozma, I mean, Nebby can become either Solgaleo or Lunala and the other one appear via Ultra Wormhole, perhaps alongside a missing Mohn. And if Necrozma appears, Yellow already has experience dealing with Psychic Pokémon, such as Mewtwo and Deoxys, not to mention Celebi and Jirachi. When Necrozma goes Ultra Burst, she'll definetily have flashbacks of fighting Lance in Cerise Island! Perhaps that way, her Eevee might become an Espeon and "befriend" with Red's Espeon.
Let's not forget that Yellow also has a Raticate and a Golem, Pokémon who happen to have Alolan Formes, so perhaps they can pull of some strings here and there to tie things up (maybe with Samson Oak - she still has Samuel's Kanto Dex - which was Red's). Not to forget that there's also Alolan Raichu (who's also part Psychic) and Pikachu's exclusive Z-Crystals (perhaps she can create them with her Viridian Forest powers on random Electrium-Zs) and in regards to Zygarde, I can see an biosphere motif with them!
Honestly, there are various routes to go with Yellow! Being a mangá OC makes her unique, versatile and unpredictable in regards to her development, I can see her being in her 20s now by this time, and if I see a wedding ring on her finger (maybe in the epilogue of the chapter, in a little chip with Kukui and Burnet) I'll absolutely flip the table!
