Not necessarily what I think they should've been for balance purposes, but what I think they could've been based on their designs and/or movepools:
Charizard (Fire/Dragon)
Blastoise (Water/Steel because the cannons)
Psyduck and Golduck (Water/Psychic)
Seel (Water/Ice)
Cloyster (Water, Water/Rock, or Water/Poison. I love it as a Water/Ice-Type, but it doesn't really look icy at all.)
Happiny, Chansey, and Blissey (Fairy)
Kangaskhan (Ground/Fighting?)
Elekid, Electabuzz, and Electivire (Electric/Fighting since it's humanoid and learns punching moves)
Pinsir (Bug/Fighting)
Gyarados (Water/Dragon, even if it's visually based on a wind sock)
Dragonair (Dragon/Flying since it can fly in the anime and even has "air" in its name)
Croconaw and Feraligatr (Water/Dark)
Noctowl (Psychic/Flying)
Ampharos (Electric/Dragon)
Azurill, Marill, and Azumarill (Water. I'm guessing Fairy was only added for balance because I see nothing fairy-like about them.)
Misdreavus and Mismagius (Ghost/Psychic)
Dunsparce (Bug/Ground)
Snubbull (Normal/Fairy)
Granbull (Normal/Dark. I get Fairy for Snubbull since it looks like someone dressed a bulldog in a fairy tutu, but not so much Granbull.)
Stantler (Normal/Psychic)
Lugia (Water/Dragon)
Celebi (Fairy/Grass)
Masquerain (Bug/Water, and change its Dex entry to like water)
Vigoroth and Slaking (Normal/Fighting)
Mawile (Dark/Steel or Dark/Fairy)
Volbeat and Illumise (Bug/Electric)
Torkoal (Fire/Rock)
Grumpig (Psychic/Dark)
Trapinch (Bug/Ground)
Vibrava and Flygon (Bug/Dragon. If Nincada can lose its Ground typing when it evolves, so can Vibrava.)
Altaria (Normal/Flying)
Milotic (Water/Fairy)
Banette (Ghost/Dark)
Absol (Dark/Psychic, since it can predict disasters)
Huntail (Water/Dark)
Gorebyss (Water/Psychic)
Groudon (Ground/Fire. At least Primal Groudon got Fire.)
Jirachi (Fairy/Psychic)
Piplup, Prinplup, and Empoleon (Water/Ice)
Staraptor (Fighting/Flying)
Luxray (Electric/Dark)
Cherrim (Grass/Fairy)
Ambipom (Normal/Fighting)
Lucario (Fighting/Psychic)
Uxie, Mesprit, and Azelf (Fairy/Psychic. They're even named after types of fairies: a pixie, a sprite, and an elf.)
Cresselia (Psychic/Fairy)
Manaphy (Water/Psychic)
Serperior (Grass/Dragon)
Dewott and Samurott (Water/Fighting)
Audino (Fairy or Normal/Fairy)
Petilil and Liligant (Grass/Fairy)
Darmanitan (Fire/Fighting)
Dwebble and Crustle (Rock or Rock/Water)
Gothita, Gothorita, and Gothitelle (Psychic/Dark)
Stunfisk (Water/Electric. I think I read somewhere that it was changed during development due to balance issues.)
Braviary (Fighting/Flying)
Meloetta (Fairy/Psychic and Fairy/Fighting)
Flabébé, Floette, and Florges (Grass/Fairy)
Helioptile and Heliolisk (Electric/Steel)
Goomy and Sliggoo (Water)
Goodra (Water/Dragon)
Rockruff and Midday Lycanroc (Rock/Normal)
Midnight Lycanroc (Rock/Dark)
Fomantis and Lurantis (Bug/Grass)
Stufful (Normal, only gains Fighting once it evolves.)
Tsareena (Grass/Fighting)
Comfey (Fairy/Grass)
Solgaleo (Fire/Psychic or Fire/Steel.)