I really don't get why people complain that new type(s) would make the game more complicated. Personally I think it'd be simple. XYZ type is good against [this] and bad against [that]. Print out a type chart if you have trouble ;p
Of course there's the metagame complication, but it's not like the metagame stays the same. In a new generation pokemon get a new ability/move/niche that makes them better (see Cloyster this gen). Between generations the metagame will change - this are banished to ubers (Blaziken comes to mind). If a new type was introduced next gen it wouldn't be that chaotic - there would be many other things that shake up the metagame. If you disagree, compare gen IV and gen V . . .
Of some of the types suggested, light and sound are definitely ones that I've heard before. But they're still nifty IMO. Light seems obvious to me: SE against dark, ghost, maybe poison, NVE against grass (plants love light), light, and maybe steel (metals can reflect light!). Could be weak against dark and ghost (I see them as opposites; don't start with me what a dark type is/isn't. I already read that).
Sound type seems a bit trickier to me. SE against normal (it has to be SE against something), water (if I'm correct, sound is faster in water), flying (ala air, sound travels in air too). NVE against steel (machines don't have ears), rock (rocks don't have ears), light (light faster than sound). Weak against light . . . uh, I'm running out of ideas!
But I am open to new types
Sound is merged with normal type that would make meloetta,whismurr and maybe jigglipuff sound types.. that's would be weird..
so sound exists.. (see: special normal moves and soundproof abillity)
of course bug buzz is also a sound move so are some others it would make bug types lose their best STAB..
Light type while seems right in the type chart.. what will you base light pokemon on? will clefable and co. types will change?
Light is represented by: electric,psychic and fighting.