Indeed. To me, if I'd add any new types to games such as these, I'd think about their function first, try to imagine it their game mechanics, think about their possible characteristics, as well what it actually represents physically. That is the most important when adding a type. Sure, there are existing types in the games that one thinks should have been molded together, and I agree by a point, until said types have defining characteristics that deserves to make them a seperate type.
So Sound could work pretty well, because it has its own characteristics, the function is obvious and the game mechanics are pretty much there by default as shown in past games. Light-type does not have these characteristics, because Light-type, in my view is already part of the Pokémon universe, representing their life-force/energy. This is proven in speculative theory because its covered in several typed attacks such as Dazzling Gleam, Solarbeam, Flash Cannon, Swift, Judgment, etc, all of which are Fairy, Grass, Steel and Normal respectively.
Aside this, yeah, I think its pretty sure we will atleast get to see the Box Mascots. For the starters, I'm not sure yet.