Honestly? Given what we have, speculating a new type is fairly understandable. New types can be still up in the air. For the first time, we have what seems to be a single eeveelution being introduced (this, too, is speculation--but they're certainly making Ninfia the center of the promotion for this eevee short). Not only that, but it's type is already proving nearly impossible to place, which puts it starkly at odds with the pattern of other eeveelutions. They're all almost immediately identifiable, and GF is specifically hiding its type. A brand spankin' new eevee evolution does sort of present the perfect opportunity to introduce a new type.
"Dark" isn't "dark" in Japan, but "evil." "Light" could still fit the bill perfectly. In a semiotic sense, "light" is an archetypical representation of "goodness" or "purity." Any of those words (or divine, or celestial, and on and on) could all be interchangeable and work perfectly well at suggesting a new and fairly cohesive and clear type--certainly as conceptually clear as "bug" or "dragon" or "steel" or "poison" or, yes, "dark." I mean, it makes sense what constitutes a bug or dragon or steel pokemon, but in terms of moves it makes no sense, and GF mostly wings it (X-Scissor is bug type! Flash Cannon is Steel!)
Most of these types aren't interpreted in a single way, either. Dark types are suggestive, yes, of darkness or shadows--Umbreon, for instance--but also sometimes suggestive of the "evilness" or at the very least the impishness suggested by the Japanese name for the type--your Weaviles or Hydreigons or Zoroarks. Poison has been interpreted as venom--Toxicroak/Arbok, but also as various forms of pollution. I just wouldn't debunk the possibility of a light type just yet.
The whole chivalry/honor thing makes sense to oppose "dark" or "evil," but so would a pure/light/divine type, just as it makes sense why fire and water have the relationship they do, and why fire and rock have the relationship they do. Because any type that exists, in practical terms, has no single foil. In practical terms, each type has a specific interaction with 16 other types. And while the relationship between Dark and Fighting can be pretty easily explained/interpreted, anybody who honestly thinks that on a thematic/conceptual level that Dark or Evil (either way you want to refer to or think about it) doesn't have a more direct and clearly implied opposite than "fighting" is stretching credibility. And the idea that Japanese culture isn't as preoccupied with dark/light-evil/good schema doesn't do much to convince me. That may very well be true, and they may see things with greater complexity and ambiguity, but these are still ultra-basic, pretty much universally recognized and understood narrative and mythic motifs.
They *did* use Dark and Steel to help nerf Psychic, sure. But they could have done that a lot more easily by just tinkering with the type chart. Sort of like how, you know, Ice suddenly (and sadly, though logically) gained another type resistant to it between Gen I and Gen II--fire. Not to mention, again, those two new types also unavoidably interacted with every other type in the game and changed things for those types. If they were added just to nerf Psychic, well, there's many much easier solutions GF could have implemented. How about they did it partly because it was--this may be crazy--fun and interesting? And it's been a long time since we've had a new type. I'd, for one, welcome it. It just switches things up and adds a new layer of complexity and opens up things aesthetically for more varied pokemon designs.
Well look, types can be become even more balanced. Not much people use Ice bug typing pokemon in competitive play, for instance. Sure, Ice is an offensive necessity, so I suppose in that sense you could try to argue there's a balance, but the point of pokemon should really be to be able to battle and win with the pokemon you love, so for me, that defines balance as making as many pokemon as possible from all types as competitively viable as possible. Obviously, it's impossible to completely balance things, but the great disparity in type allocation among the tiers is pretty damning evidence to indicate that things could be *more* balanced. Sorry, this has become longer than I hoped, but, tl;dr, my opinion is that I wouldn't say that pokemon doesn't need new types. Not yet anyway.
