This should be interesting, though looking up your other thread, I'll be using some of my other ideas.
Ash/Satoshi:
Seeing as I think he has enough Pokémon already, I don't want him to capture a single one more. But on the topic of a filler arc, here are a few a think would work;
Numel - A cool fire type, a way to promote R/S/E, plus I don't know why but I think it would work really well with Ash, I can see one with him so easily.
Aron > Lairon - Ash hasn't owned really ever owned a steel type, and I think it would be pretty cute with him.
I can see at least two members of his old Hoenn team join back up with him to promote R/S/E remakes.
Iris:
Deino - If or when Axew does evolve, I could see her catch a Deino after, it could be like Axew in personality, and sort of want to become a strong Hydreigon or something.
Mudkip or Torchic - I can see Iris somehow get one of these starters, for promotion, like when May got a Squirtle.
Kirlia - Most probably caught as a Kirlia, I don't know why, but I can just see it. :x
Swablu > Altaria or Bagon - I can see her at least have one the these two, since she is a dragon type trainer after all, I can't see her with the both though. I'm leaning towards Swablu because I think Altaria is a bit of a different dragon type Pokémon, one she won't be used to.
Girafarig - Might sound a little random, but I can see her with it, I could see it's tail being a comedic point.
Snorunt > Froslass - Idk, maybe parallel Ash's Glalie. Evolve if she carries on with Ash after east unova.
Cilan:
Cubchoo - It could turn out to be like the Togepi/Happiny/Bonsly of Unova, Cilan could take care of it like a baby or whatever. Doesn't necessarily have to evolve. Who knows, he might even encounter it in BW48.
Shuppet - Help him get over his fear of ghosts. Poor guy