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.
I'd have to disagree that there aren't enough Pokemon to be Light type. The ones I listed initially are just a handful that had light potential, particularly things that are relatively useless to use as they are currently like Lumineon, Sunflora & Ampharos, plus a lot of the fairies could easily be Light type, Clefairy of the moon, Gardevoir, even Sylveon before Fairy type was announced. I also don't think it would have been strange (if light type was real) for it to be effective against dragons. Not all of the match up resistances and what not that Fairy has been given are particularly logical or obvious. For example, what have fairies got to do with steel? In mythology dragons could be slayed by humans, so perhaps Normal or Fighting type should be effective against dragon if we're always going by those kind of rules. Like you say, Fairy was mainly brought in to neutralise all the overpowered dragons, so I think light could have just done exactly the same.
I could live with poison on Huntail, and I still think giving Kangaskhan and Ursaring dark (with the actual situation of the typings) would be a good boost for them, especially Kangaskhan as it's so neglected at the moment. As I said before though, perhaps the mega evolution will be enough to give it a bit more popularity this time round. You're right, Mr. Mime should never go to Normal, it would be pointless, but I think adding Fairy to the Psychic would be good just because him (and Mime. Jr) give of that slightly feminine, mimicky, mythological or magical vibe.
Beartic's design is quite bad I think. I would have preferred to see it geared towards a cute but powerful polar bear as opposed a small headed polar bear dribbling ice...
As for Luxray, yes it's a tricky one whether it should or it shouldn't, I think if it did it should only be Luxray and not Luxio or Shinx. I think it's design is crying out for a Dark sub type though, and I mean some of the fan art is very geared towards dark. And I'm not really sure what you mean about seeing through walls. You're saying it's radiation created by electricity, but I'm sure it's not actually a possible thing in real life for any animal? And also I don't recall Luxray ever being said to have 'x-ray' vision specifically, just that it has that ability to find prey by seeing through walls (correct me if I'm wrong). That's why I'm saying it's a dark trait, because it's unnatural and kind of creepy, and suggests it has a supernatural ability to some extent.
As for your suggestions
Definitely agree with you on Psyduck like in my first post
Drowzee and Hypno - Perhaps. I mean I would refer you back to your argument against Pinsir becoming Fighting type, I wouldn't say the Hypno or Drowzee look anywhere as near built for fighting as Pinsir does, but if you look at the facts it does have access to a few fighting moves and the elemental punches, so maybe, but I doubt they would do it, although Hypno does have kind of bulky limbs.
Cubone & Marowak - I would have no objection to this. Wearing your mothers dead skull as a mask is very dark indeed.
Jynx - for me it's kind of one of those that has fans and haters, I wouldn't touch it. It has definitely developed some interesting concept art for an evolution/mega evolution.
Please don't touch Mew, it's too much of a classic oldie to change!
Sentret/Furret - Not for me. It's kind of that token Normal rodent you get at the beginning of every game, although I suppose we have seen sub types been given for them before (Bibarel). I don't think ground though as I couldn't see Furret dishing out an Earthquake... it wouldn't really have much beyond Dig & Mud Slap.
Bellossom - I agree could become part Fairy. Another example of something that could have been ideal for Light type though!
Octillery - would be interesting as it's not very popular. I think I had one on about my 4th run through of Pearl not too long ago and it was actually quite good in-game.
Dunsparce - Definitely needs love, a mega evolution would be nice, but I do think it does a great job of that being token useless pokemon in the Johto region.
Stantler - Normal/Psychic would be ideal to spice this thing up a bit, I never really got how Girafarig got the Psychic/Normal typing and this thing didn't. It doesn't have the movepool to support an Ice sub type as it can't learn a single Ice type move though, and if it was given grass it would just be another Sawsbuck.
Celebi - I completely agree with you but I don't think they'd touch it as it's had movies built around it and other commercial attention etc.
Slaking - I disagree purely because it's Norman's trademark pokemon who is the Normal type gym leader in Hoenn. But with that said Whitney has a Clefairy, which I'm sure will now become Fairy type, although it was not her main Pokemon. Also we still need some good Normal types now that the Fairy type is being brandished and the only ones I can think of from the top of my head are Slaking, Granbull, Stoutland & the Porygon's.