Well, I'm gonna make a travel companion from scratch since I like fun. This is my idea of perfect, so keep in mind, this is just an opinion!
Name: Mira!
Appearance She'd have medium length pink hair, tied in braids, blue eyes, with dark blue glasses over them, and wear the X/Y style sundress. I think it'd be interesting if she were about two inches taller than Ash, just for the fun of it. I think she'd have a
Personality: She'd be a slightly ditsy oujo-sama style character. However, beyond the surface, unlike most oujos, she doesn't mind getting dirty, and would be kind overall. Loves fashion like Dawn did in her run. I don't know if the oujo works for Ash, but I think it'd work for him.
Goal: She'd battle, of course. I want a girl who's goal is to do battles, rather than contests/become a master of a certain type. She's probably prefer a certain battle style like inverse battle, maybe? (which never got showcased in the anime anyway. Rip...)
Pokemon: Fighting, Psychic and Fairy type Pokemon. :D
I may reply again later with previous characters from the show and how I feel they'd be if they were Ash companions~.