So just because light is caused by fire and electricity it can't have it's own type? By the same standard then, should ice not be abandoned as it is just another form of water? It's not as if it wouldn't fit in. We've seen moves like Bite changed to Dark, so I see no problem with minor shifts in moves. I think solar beam could stay as grass anyway because solar means sun, which consists of FIRE just as much as it is to do with light and no one ever complains about that. Synthesis wouldn't be a problem either as photosynthesis is carried out by plants (grass). I think in the world of Pokemon, Light just would have fitted in perfectly, both for Pokemon typing, and for in-game balance. It could have came in with essentially the same effectiveness rules as fairy, and could have been assigned to essentially the same Pokemon with the exception of a few. Plus, as a general rule I feel there should be light if there is a dark. Light is a separate element to fire and electricity, for example, in Greek mythology there is Apollo God of the sun and light and Zeus God of the skies and thunder, and Hephaestus God of Fire, so in the same respect there could well have been Pokemon of Light, Pokemon of Fire & Pokemon of Electricity all separately and no one would have blinked an eye.
As for my suggested typings
Perhaps Kangaskhan being dark is more something I want than should happen, I only included on that list as, for a Kangaroo, it is quite sinister looking, with teeth on show, the dark helmet feature, and has access to attacks obviously like Crunch and Sucker Punch. I don't see how it's any less of a dark looking pokemon than Tyranitar or Hippowdon but I can live without this changing. I think it would benefit Kangaskhan competitively too.
Ursaring, yes the moon symbol, the dark moves, the sinister looks, the fact you find it in caves, the fact it reminds me vaguely of the new dark type panda pokemon as well. I just think it would give it an extra edge.
As for Mr. Mime, another one that could do without it, I'll admit. Yes it is based on a circus clown, but really does a circus clown point to psychic, or any other type in the game? Maybe normal. Mr. Mime is a kind of feminine looking, spritey looking like a weird little magic man, with pink on him, that's why perhaps I thought he might fit nicely into the fairy type. I do not believe this is on the same scale of ridiculousness as Marill, the water mouse, being branded as a fairy.
For Pinsir, I don't understand how you can say it doesn't strike you as a fighting type, it has access to a large amount of fighting type moves, and it looks more aggressive than Heracross looks - who is actually a fighting type. It has long arms similar to Heracross too with claws and those mahoosive pinsirs on it's head for fighting as well. I wouldn't mind seeing as a Steel type but it has no real Steel type moves would just be second rate to Scizor whereas Pinsir in the same boat as Heracross is more interesting. I think it just screams fighting type when you consider how it looks.
I agree with you that Huntail should receive another type as well, I can't see Steel myself but perhaps Dark/Ice/Poison/Rock/Fighting? It's too plain looking really, just looks like an aggressive fish. Maybe just leave them both alone and give Gorebyss access to a couple of Fairy attacks.
As for the Luxray line, I fail to see how you can say it's like putting Manectric in the Dark category? Manectric is light blue and it's head looks like it's made of thunderbolts. Luxray is dark and only blatant electric resemblance is its tail and perhaps the stripes on it's front legs. I think there are a lot of people out there who would like to see Luxray as a Dark type. It's appearance makes it perfect for Dark type, and also it can see through walls which is kind of a 'dark' trait. It would give it more depth and STAB on Crunch too. I think the thing looks more dark than Absol, the only thing is that it's based on a Lynx, which means light, but then again that's only because of their light eyes.
Well, I have to admit that you've got a strong point there but still. I don't feel like Light-type would have been the right way to go as it would have clearly been counter part for Dark-type then. And what we've seen so far Fairy-type was to be introduced to balance all those Dragon-type pokémon who are way too overpowered. And so far light isn't anyhow mythological to me atleast. Don't get me wrong, I don't hate Light-type but at this moment there really aren't many Light-type looking pokémon even in games. Most that would fall under that category would propably be Electric- and Fire-type pokémon, maybe some Dark-type even (for example Umbreon and Espeon who are daylight pokémon and can use Moonlight etc.) and a couple of Bug-type pokémon too.
And about Ice-type, I wasn't really keen on that either for that obvious reason that it's basicly water in a different form. But still, Ice-type doesn't only count water that's frozen or turned into snow, there's also the temperature aspect as there are freezing winds and stuff like that which Ice-type pokémon can use.
Well, for a Kangaroo I have to admit that Kangaskhan is way menacing and scary but maybe going Dark-type wouldn't be the right choice still. It still needs a power boost though through a new evolution or retyping.
At this point retyping Ursaring to Normal/Dark-type would basicly just make it almost like a counter part to Pangoro. I hate myself for saying this but Light-type would have been pretty fitting for Ursaring.
...and about bears, what do you think about Beartic? I think they should really do something about it, maybe give it an evolution and retype it to something else than just Ice-type because now it's got way too many weaknesses in my opinion.
Mr.Mime doesn't really look like Psychic-type either to me but I assume retyping it to pure Normal-type would just make it even more useless than what it is now. If they gave it an evolution then I could see it going Psychic/Fairy.
It's just that I feel like adding Fighting-type to Pinsir would basicly make it even more comparable to Heracross which annoys me a lot already. I don't feel like Heracross should be partly Fighting-type either as it has no real traits of a fighter in it. It's just a freaking beetle to me. But I do feel that Pinsir needs some love and quickly because it's a great offensive pokémon who just suffers from being monotype.
Steel-type just came out strongly to me but honestly I find Huntail going Water/Poison-type much more better. It has that sting looking tail for example and those teeth, plus it would totally help it as in competitive aspect.
Okay, maybe Manectric wasn't the best example but still, I just don't see at this point going to Dark-type. I don't say it would be anyhow bad that it would get retyped to Electric/Dark-type, It would actually help it a lot and make it really useful and fearsome in competitive aspect. Personally I just want to keep it as Electric-type atleast for now.
Oh and Luxray seeing through walls has nothing to do with Dark-type. It's been stated that it can utilize x-ray vision in it's eyes to see through walls and as you might know x-ray vision is basicly radiation that is fueled by electricity for example.
Here's some pokémon that came in mind that could need some love and might need to retyped:
Psyduck/Golduck: Water/Psychic
Drowzee/Hypno: As odd as it sounds, I could see them getting retyped to Psychic/Fighting for example.
Cubone/Marowak: Either give them an evolution or change them to Ground/Dark-type. After all they tend to reside in caves mostly so it would be fitting for them.
Jynx: Well, maybe it doesn't need to be retyped necessarily but it need some love too. Maybe an evolution that would look more or less like Nicki Minaj perhaps?
Mew: Psychic/Fairy
Sentret/Furret: Oh God, I love these two so it just kills me that they are... well, useless. Retype them to Normal/Ground for example and I'd be thrilled!
Bellossom: Actually, it really doesn't look all that fairy-ish to me to begin with but somehow I wish they retype it to be Grass/Fairy. It would help it a lot.
Dunsparce: Seriously guys, this pokémon has so much potential in it but they've completely forgot it. Retype it, evolve it, just give it some love!
Octillery: Water/Poison - it's on an octopus for Christ's sake, they tend to use poison to catch their prey.
Stantler: I know it's a reindeer and it can use pheromones to create illusions and it lives near people which is why I could see it going multiple ways as Normal/Grass, Normal/Psychic or even Normal/Ice.
Celebi: Definitely Grass/Fairy even though it would be really weak to Fire-type then. It just looks really fairy-ish to me plus retyping it Grass/Fairy would totally make it stand out and bring it back from the shadows.
Slaking: Make it like, Normal/Fighting and give it another ability so it would be more useful than what it is now. It has huge potential in it and now it's ability basicly ruins it completely.