Sigh... I wish that Diantha had a different Mega-Evolving Pokemon instead of Mega Gardevoir to use as her signature/ace Pokemon, because I always associated the Ralts line with Wally.. I kind of wish a Mega Gallade could be introduced, though (since Wally's particular Ralts is male), so that Wally is heightened in power and the Ralts line gets "balanced" again in terms of evolutions.
Also, I think
"Battle! VS Rival (Hoenn)" should play during Wally's battles, since he kind of assumes the rival role after Brendan/May essentially abandon it (unfortunately) at Lilycove City.
I feel the same way about Diantha and her Mega Gardevoir. While I feel it fits her, it seems odd for Wally to have a Mega Gardevoir while that's Diantha's ace (only because Mega Evolution is so rare within the games; if it was more common I'd say it's more acceptable for some overlap).
I'd love for Mega Gallade, but I'd also like for Wally to retain the Ralts he actually catches (for those rare times he gets a shiny, mainly, ha ha!), and then it would feel weird if he had either Mega Gallade, or Mega Gardevoir like Diantha...it's really tough to say!
A major point of Wally's character is that he's always worse than you, but he's still inspired to continue on...so while one may argue he just won't get a Mega Pokémon at all, I would feel very bad! He is clearly able to bond with his Pokémon even if he's not the best battler.
As for the music...oh yes! In HGSS, Lugia and Ho-oh got their own music when they originally had none, and the music used for Entei, Suicune, and Raikou was updated to have their own flair for each individual Pokémon; ORAS might do something similar for its legends, but I'd
much rather have it applied to its Trainers! I've heard people wanting Maxie and Archie to have individual battle themes (but that's a whole other discussion), but I definitely want Wally to either gain a new theme altogether, or share the theme with Brendan/May, just don't give him the regular Trainer theme--he deserves better!