It is rare when gamefreak will retype species that have already been around for a few generations. In fact, they have only done it twice, and only in lieu of adding new types. Still, I can't help but feel there are more than a few missed typing opportunities. Here we can discuss these and their competitive and even their aesthetic benefits.
Here is the list that I have off of the top of my head:
BLASTOISE
Oh, sad, forlorn Blastoise. When Gen 2 brought us the Steel type, a certain magnet Pokemon was given the blessing of the first type change, while Blastoise was left feeling kind of awkward. I mean, LOOK at it. With its metallic cannons and certainly a rigid shell, can you imagine it is made up of anything except steel? Sadly, the typing would have made it a competitive beast long before the dawn of Mega evolutions. Which, incidentally, STILL neglected to give him a metallic dip. Sigh.
PRIMAPE
I don't have as much to rant about here, but it always struck me as if it could pull off being a Fighting/Dark type. Sadly, this does not seem to have been considered. Moving on.
FERALIGATR
Why, oh why is this NOT a Water/Dark type? Then again, I suppose they wanted to just stick to monotyping that group of starters. Still, a sad excuse to not give the Big Jaws Pokemon a STAB Crunch.
FLYGON
I have always felt, aesthetically, that this should be Bug/Dragon. Then again, its current typing offers more advantages, and it would lose the Sandstorm immunity. So this one is debatable.
MILOTIC
I don't know where his mega is, but it had darn well make up for the serious lack of Fairy typing.
MISMAGIUS
One of my favorite Pokemon, and I have to say... WHAT were you thinking, Gamefreak??? This one should be Ghost/Fairy! It is the "Magical" Pokemon, after all! Plus that typing would expand its competitive utility and STAB coverage by so much. I cannot express well enough my vast disappointment.
ABOUT HALF OF GEN FIVE
Gen 5 irritated me more than any other with its typing choices. So many missed opportunities, so many that just feel aesthetically awkward. And, of course, my chiefest complaint is with the starters. AHEM.
Are you all ready for this?
SERPERIOR: Should be part Dragon.
EMBOAR: This made a lot of people mad besides myself. Because reasons. This should be Fire/Ground.
SAMUROTT: Let's see... has armor and swords made from shells, draws a lot of inspiration from samurais, has a code of honor akin to warriors... yup, this is pure Water!!! ....No. This deserves to be Water AND Fighting type. Enough said.
AUDINO: Not being a Fairy, I can live with. But still, it feels a little off.
COFAGRIGUS: Woulda made a neat Ghost/Steel, especially with all of the Pokedex entries detailing its GOLD and METAL body.
GOTHITELLE: Should be Psychic/Dark. I mean, just look at it! That would feel so right! Plus, it learns a TON of Dark type attacks naturally! Plus that typing would have been such a boon. What more can I say?
GARBODOR: One that I am most angry about, more than any of the above. If I could only choose to retype one, it would be him. He deserves to be our first Poison/Steel type, as he IS a trash heap, with obvious metal pieces making up his body. The typing would be great defensively, especially if they gave him 10 or 20 more base Def.
There are honestly a few others from this gen, but they are mainly minor gripes.
Moving on...
From Gen 6, I feel they did an almost perfect job with typing. I only have a few small nitpicks...
AROMATISSE: Hmm... uses strong chemical gases and has a mask reminiscent of plague doctors. Yup, this should be a Fairy/Poison type. Would be pretty neat defensively and offensively.
HELIOPTILE and HELIOLISK: Imagine the implications if these guys truly lived up to the sun-powered gimmick and were Fire/Electric?? With Dry Skin, it would have been immune to water and a bigger beast in the sun with Solar Power. Actually, it would be able to dominate three of the four weather types this way. Its Base Stats would have stopped it from being too OP with that typing, which I feel is perfect.
XERNEAS: Fairy and Grass would have given this whole trio some dual typing, and it would have evened out the playing field between Xerneas and Yveltal. Plus, its design and mythos draws a lot of inspiration from the Norse myth of the World Tree, which would befit the typing even more.
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Well, that is all I have for now. If anyone out there has anything to say add, I would be happy to hear it.