The Type wasn't revealed until much later than Gen2, but I feel that Celebi should've got Grass/Fairy instead of staying Grass/Psychic. It's movepool would probably need a makeover, but it looks more like a Fairy than a lot of actual Fairy-Types! I feel that Chansey and Blissey should've got it as well (pure Fairy or Normal/Fairy like the Jigglypuff line), but I'm sure they had their reasons.
As far as Types that
did exist as of Gen2...
- Croconaw/Feraligatr: Water/Dark
- Noctowl: Psychic/Flying
- Ampharos: Electric/Dragon
- Sudowoodo: I feel that Rock/Fighting could've worked, but I'm fine with pure Rock
- Misdreavus: Ghost/Psychic
- Pineco: Bug/Grass (since its shell is made of tree bark)
- Dunsparce: Bug/Ground
- Gligar: Poison/Flying (though Ground/Flying is a lot cooler)
- Granbull: Dark (Dark/Fairy for modern games)
- Remoraid/Octillery: Water/Dark (somewhat fitting, being based on artillery)
- Stantler: Normal/Psychic
- Elekid and family: Electric/Fighting
Magby and family: Fire/Dragon (with a redesign of Magmortar to better reflect the Type)
- Suicune: Water/Ice (I would say pure Ice, but that would throw-off the legend of Ho-Oh and the revived beasts)
- Lugia: Water/Flying or Water/Dragon (they even made it a Water-Type in the TCG!)