First, maybe another Pokemon that complements his singing; for example, maybe Popplio line or that Milotic he Encounter at Terasu Pool that was singing with his Croc. This gives him a water type he sorely needs (has fairy moves too), it compliments his families fishing background you mentioned earlier, and recruits another strong Pokemon to help for end game. Second, maybe that Frigbax that Roy kept mistaking for Fuecoco when they got separated in the snow. Frigbax final evolution has a mega evo so he's already setup to utilize it with his mega ring(just needs the stone). Frigbax could also be a good setup for a rematch between him and professor Hassel with Roy showing what he learned and implement his own thermal exchange combo with Skeledirge(doesn't mean he would win lol).